Immunity and Immunizations

In 2010, a pertussis (whooping cough) outbreak in California sickened 9,143 people and resulted in 10 infant deaths: the worst outbreak in 63 years (Centers for Disease Control 2011b). Researchers, suspecting that the primary cause of the outbreak was the waning strength of pertussis vaccines in older children, recommended a booster vaccination for 11–12-year-olds and also for pregnant women (Zacharyczuk 2011). Pertussis is most serious for babies; one in five needs to be hospitalized, and since they are too young for the vaccine themselves, it is crucial that people around them be immunized (Centers for Disease Control 2011b). Several states, including California, have been requiring the pertussis booster for older children in recent years with the hope of staving off another outbreak.

But what of people who do not want their children to have this vaccine, or any other? That question is at the heart of a debate that has been simmering for years. Vaccines are biological preparations that improve immunity against a certain disease. Vaccines have contributed to the eradication and weakening of numerous infectious diseases, including smallpox, polio, mumps, chicken pox, and meningitis.

However, many people express concern about potential negative side effects from vaccines. These concerns range from fears about overloading the child’s immune system to controversial reports about devastating side effects of the vaccines. One misapprehension is that the vaccine itself might cause the disease it is supposed to be immunizing against.

Another commonly circulated concern is that vaccinations, specifically the MMR vaccine (MMR stands for measles, mumps, and rubella), are linked to autism. The autism connection has been particularly controversial. In 1998, a British physician named Andrew Wakefield published a study in Great Britain’s Lancet magazine that linked the MMR vaccine to autism. The report received a lot of media attention, resulting in British immunization rates decreasing from 91 percent in 1997 to almost 80 percent by 2003, accompanied by a subsequent rise in measles cases (Devlin 2008). A prolonged investigation by the British Medical Journal proved that not only was the link in the study nonexistent, but that Dr. Wakefield had falsified data in order to support his claims (CNN 2011). Dr. Wakefield was discredited and stripped of his license, but the doubt still lingers in many parents’ minds.

In the United States, many parents still believe in the now discredited MMR-autism link and refuse to vaccinate their children. Other parents choose not to vaccinate for various reasons like religious or health beliefs. In one instance, a boy whose parents opted not to vaccinate returned home to the U.S. after a trip abroad; no one yet knew he was infected with measles. The boy exposed 839 people to the disease and caused 11 additional cases of measles, all in other unvaccinated children, including one infant who had to be hospitalized. According to a study published in Pediatrics (2010), the outbreak cost the public sector $10,376 per diagnosed case. The study further showed that the intentional non-vaccination of those infected occurred in students from private schools, public charter schools, and public schools in upper-socioeconomic areas (Sugerman et al. 2010).

Consider these questions about immunization:

  • Should parents be forced to immunize their children?
  • How does this story of vaccines in a high-income region compare to that in a low-income region, like sub-Saharan Africa, where populations are often eagerly seeking vaccines rather than refusing them?
  • Do you believe all children should receive vaccinations?


Immunity and Immunizations

Shots may hurt a little... but the diseases they can prevent can hurt a lot more! Immunization shots, or vaccinations, are essential. They protect against things like measles, mumps, rubella, hepatitis B, polio, diphtheria, tetanus and pertussis (whooping cough). Immunizations are important for adults as well as for children. Here's why.

Your immune system helps your body fight germs by producing substances to combat them. Once it does, the immune system "remembers" the germ and can fight it again. Vaccines contain germs that have been killed or weakened. When given to a healthy person, the vaccine triggers the immune system to respond and thus build immunity.

Before vaccines, people became immune only by actually getting a disease and surviving it. Immunizations are an easier and less risky way to become immune.

Optional Learning Activity

Take action to raise awareness of immunizations (e.g., Assist a school nurse to host an immunization education event for parents at a local school)



Vaccines

Vaccines are the best defense we have against serious, preventable, and sometimes deadly contagious diseases. Vaccines are some of the safest medical products available, but like any other medical product, there may be risks. Accurate information about the value of vaccines as well as their possible side-effects helps people to make informed decisions about vaccination.

Federal law requires that Vaccine Information Statements explaining vaccine benefits and risks be handed out whenever (before each dose) certain vaccinations are given. Vaccine Information Statements are available in Spanish and many different languages. In addition, more detailed information describing the benefits and risks of a particular vaccine is available in the Prescribing Information from the Food and Drug Administration.

The following sections answer common questions asked about vaccines and how vaccines are tested and monitored to ensure they are safe and effective. These sections are currently available in English only.

Vaccines are held to the highest standard of safety. The United States currently has the safest, most effective vaccine supply in history. Vaccines undergo a rigorous and extensive evaluation program to determine a product’s safety and effectiveness. If a vaccine does receive approval, it is continuously monitored for safety and effectiveness.

Many partners work together to make sure vaccines are safe. Government health scientists work with manufacturers, health care providers, academia, and global health groups such as the World Health Organization to build a comprehensive vaccine safety system. At the Department of Health and Human Services, primarily three agencies work on vaccine safety:

  • Centers for Disease Control and Prevention (CDC)
  • National Institutes of Health (NIH)
  • Food and Drug Administration (FDA)


Vaccines undergo rigorous and extensive testing to determine their safety and effectiveness prior to approval. Following approval, FDA carefully monitors the quality of vaccines—all manufactured lots must pass tests before they can be used. Vaccine manufacturers also must follow strict manufacturing standards, and FDA conducts routine inspections of manufacturing sites.

Scientists from FDA and CDC work closely to monitor reports of vaccine side effects (adverse events) after they are approved and used widely. FDA and CDC take all reports seriously, and work together to evaluate and address any potential problems.

Learning Activity

Watch this video titled Get The Picture: Childhood Immunizations:



After talking with parents across the country, CDC put together this short video to help answer the tough questions that real moms had about childhood immunizations.

Understanding the importance of vaccines is crucial for you to protect your children’s health. Read the script
  • Did watching this video help you decide on whether or not to get your child immunized?
  • Interview some parents to find out what they think about this issue.


Potential Side Effects

Vaccines, like all medical products, may cause side effects in some people. Most of these side effects are minor, such as redness or swelling at the injection site. Read further to learn about possible side effects from vaccines.

Any vaccine can cause side effects. For the most part these are minor (for example, a sore arm or low-grade fever) and go away within a few days.

Remember, vaccines are continually monitored for safety, and like any medication, vaccines can cause side effects. However, a decision not to immunize a child also involves risk and could put the child and others who come into contact with him or her at risk of contracting a potentially deadly disease.

How Well Do Vaccines Work?

Vaccines work really well. No medicine is perfect, of course, but most childhood vaccines produce immunity about 90–100% of the time.

What about the argument made by some people that vaccines don’t work that well . . . that diseases would be going away on their own because of better hygiene or sanitation, even if there were no vaccines?

That simply isn’t true. Certainly better hygiene and sanitation can help prevent the spread of disease, but the germs that cause disease will still be around, and as long as they are they will continue to make people sick.

All vaccines must be licensed (approved) by the Food and Drug Administration (FDA) before being used in the United States, and a vaccine must go through extensive testing to show that it works and that it is safe before the FDA will approve it. Among these tests are clinical trials, which compare groups of people who get a vaccine with groups of people who get a control. A vaccine is approved only if FDA makes the determination that it is safe and effective for its intended use.

If you look at the history of any vaccine-preventable disease, you will virtually always see that the number of cases of disease starts to drop when a vaccine is licensed. Vaccines are the most effective tool we have to prevent infectious diseases.

Prevention

Vaccines help the body’s immune system prepare for future attacks. Vaccines consist of killed or modified microbes, parts of microbes, or microbial DNA that trick the body into thinking an infection has occurred. A vaccinated person’s immune system attacks the harmless vaccine and prepares for invasions against the kind of microbe the vaccine contained. In this way, the person becomes immunized against the microbe: if re-exposure to the infectious microbe occurs, the immune system will quickly recognize how to stop the infection.

This section explains more in detail about how your immune system works to prevent you from getting sick. Knowing how your immune system works may help you understand how vaccines work with your immune system to protect you.

What is the Immune System?

The immune system is a network of cells, tissues, and organs that work together to defend the body against attacks by “foreign” invaders. These are primarily microbes—tiny organisms such as bacteria, parasites, and fungi that can cause infections. Viruses also cause infections, but are too primitive to be classified as living organisms. The human body provides an ideal environment for many microbes. It is the immune system’s job to keep them out or, failing that, to seek out and destroy them.

When the immune system hits the wrong target, however, it can unleash a torrent of disorders, including allergic diseases, arthritis, and a form of diabetes. If the immune system is crippled, other kinds of diseases result.

The immune system is amazingly complex. It can recognize and remember millions of different enemies, and it can produce secretions (release of fluids) and cells to match up with and wipe out nearly all of them.

The secret to its success is an elaborate and dynamic communications network. Millions and millions of cells, organized into sets and subsets, gather like clouds of bees swarming around a hive and pass information back and forth in response to an infection. Once immune cells receive the alarm, they become activated and begin to produce powerful chemicals. These substances allow the cells to regulate their own growth and behavior, enlist other immune cells, and direct the new recruits to trouble spots.

In addition, scientists are rapidly unraveling the genetic blueprints that direct the human immune response, as well as those that dictate the biology of bacteria, viruses, and parasites. The combination of new technology and expanded genetic information will no doubt reveal even more about how the body protects itself from disease.

Mounting an Immune Response

Infections are the most common cause of human disease. They range from the common cold to debilitating conditions like chronic hepatitis to life-threatening diseases such as AIDS. Disease-causing microbes (pathogens) attempting to get into the body must first move past the body’s external armor, usually the skin or cells lining the body’s internal passageways.

The skin provides an imposing barrier to invading microbes. It is generally penetrable only through cuts or tiny abrasions. The digestive and respiratory tracts—both portals of entry for a number of microbes—also have their own levels of protection. Microbes entering the nose often cause the nasal surfaces to secrete more protective mucus, and attempts to enter the nose or lungs can trigger a sneeze or cough reflex to force microbial invaders out of the respiratory passageways. The stomach contains a strong acid that destroys many pathogens that are swallowed with food.

If microbes survive the body’s front-line defenses, they still have to find a way through the walls of the digestive, respiratory, or urogenital passageways to the underlying cells. These passageways are lined with tightly packed epithelial cells covered in a layer of mucus, effectively blocking the transport of many pathogens into deeper cell layers.

Mucosal surfaces also secrete a special class of antibody called IgA, which in many cases is the first type of antibody to encounter an invading microbe. Underneath the epithelial layer a variety of immune cells, including macrophages, B cells, and T cells, lie in wait for any microbe that might bypass the barriers at the surface.

Next, invaders must escape a series of general defenses of the innate immune system, which are ready to attack without regard for specific antigen markers. These include patrolling phagocytes, natural killer T cells, and complement.

Microbes cross the general barriers then confront specific weapons of the adaptive immune system tailored just for them. These specific weapons, which include both antibodies and T cells, are equipped with singular receptor structures that allow them to recognize and interact with their designated targets.

Immunity

Long ago, physicians realized that people who had recovered from the plague would never get it again—they had acquired immunity. This is because some of the activated T and B cells had become memory cells. Memory cells ensure that the next time a person meets up with the same antigen, the immune system is already set to demolish it.

Immunity can be strong or weak, short-lived or long-lasting, depending on the type of antigen it encounters, the amount of antigen, and the route by which the antigen enters the body. Immunity can also be influenced by inherited genes. When faced with the same antigen, some individuals will respond forcefully, others feebly, and some not at all.

An immune response can be sparked not only by infection but also by immunization with vaccines. Some vaccines contain microorganisms—or parts of microorganisms— that have been treated so they can provoke an immune response but not full-blown disease.

Immunity can also be transferred from one individual to another by injections of serum rich in antibodies against a particular microbe (antiserum). For example, antiserum is sometimes given to protect travelers to countries where hepatitis A is widespread. The antiserum induces passive immunity against the hepatitis A virus. Passive immunity typically lasts only a few weeks or months.

Vaccines

For many years, healthcare providers have used vaccination to help the body’s immune system prepare for future attacks. Vaccines consist of killed or modified microbes, parts of microbes, or microbial DNA that trick the body into thinking an infection has occurred.

A vaccinated person’s immune system attacks the harmless vaccine and prepares for invasions against the kind of microbe the vaccine contained. In this way, the person becomes immunized against the microbe. Vaccination remains one of the best ways to prevent infectious diseases, and vaccines have an excellent safety record. Previously devastating diseases such as smallpox, polio, and whooping cough (pertussis) have been greatly controlled or eliminated through worldwide vaccination programs.

Recommended Vaccines for Young Adults

The transition to adulthood is an exciting time in a young person’s life. Starting a career, getting an apartment, entering college, or joining the armed forces all offer unique rewards and challenges.

Yet young adults may not know that some vaccines can make this transitional time a healthier one.

Vaccines recommended for young adults ages 19–24 include:

  • Meningococcal conjugate vaccine, which helps prevent meningococcal disease
  • Tdap vaccine, which protects against tetanus, diphtheria, and pertussis (also known as whooping cough)
  • HPV vaccine, which protects against the viruses that cause most cervical cancers, anal cancer, and genital warts
  • Seasonal flu vaccine


There may be other vaccines recommended for young adults because their health, job, or lifestyle may put them at higher risk for certain diseases. Young adults should talk to a doctor or nurse to find out if there are other vaccines that they may need.

Learning Activity

Take this quiz to find out to find out which vaccines YOU may need.

Antibiotic / Antimicrobial Resistance

Antibiotics and similar drugs, together called antimicrobial agents, have been used for the last 70 years to treat patients who have infectious diseases. Since the 1940s, these drugs have greatly reduced illness and death from infectious diseases. Antibiotic use has been beneficial and, when prescribed and taken correctly, their value in patient care is enormous.

However, these drugs have been used so widely and for so long that the infectious organisms the antibiotics are designed to kill have adapted to them, making the drugs less effective. People infected with antimicrobial-resistant organisms are more likely to have longer, more expensive hospital stays, and may be more likely to die as a result of the infection.

Sexually Transmitted Diseases and HIV

Sexually transmitted diseases (STDs) are infections that you can get from having sex with someone who has the infection. The causes of STDs are bacteria, viruses, and parasites. There are more than 20 types of STDs, including:

  • Bacterial

    • Gonorrhea


  • Viral

    • Genital herpes
    • HIV/AIDS
    • HPV
    • Syphilis


  • Parasitic

    • Trichomoniasis




Most STDs affect both men and women, but in many cases the health problems they cause can be more severe for women. If a pregnant woman has an STD, it can cause serious health problems for the baby.

If you have an STD caused by bacteria or parasites, your health care provider can treat it with antibiotics or other medicines. If you have an STD caused by a virus, there is no cure. Sometimes medicines can keep the disease under control. Correct usage of latex condoms greatly reduces, but does not completely eliminate, the risk of catching or spreading STDs.

Gonorrhea

Gonorrhea is a curable sexually transmitted disease. It is most common in young adults. The bacteria that cause gonorrhea can infect the genital tract, mouth or anus.

Gonorrhea does not always cause symptoms, especially in women. In men, gonorrhea can cause pain when urinating and discharge from the penis. If untreated, it can cause epididymitis, which affects the testicles and can lead to infertility. In women, gonorrhea can cause bleeding between periods, pain when urinating and increased discharge from the vagina. If untreated, it can lead to pelvic inflammatory disease, which causes problems with pregnancy and infertility. Gonorrhea can pass from mother to baby during pregnancy.

You can cure gonorrhea with antibiotics prescribed by your health care provider. Correct usage of latex condoms greatly reduces, but does not eliminate, the risk of catching or spreading gonorrhea.

Genital Herpes

Genital herpes is a sexually transmitted disease (STD) caused by a herpes simplex virus (HSV). It can cause sores on your genital or rectal area, buttocks, and thighs. You can get it from having sex, even oral sex. The virus can spread even when sores are not present. Mothers can also infect their babies during childbirth.

Symptoms of herpes are called outbreaks. You usually get sores near the area where the virus has entered the body. They turn into blisters, become itchy and painful, and then heal. Sometimes people do not know they have herpes because they have no symptoms or very mild symptoms. The virus can be more serious in newborn babies or in people with weak immune systems.

Most people have outbreaks several times a year. Over time, you get them less often and the symptoms become milder. The virus stays in your body for life.

Medicines do not cure genital herpes, but they can to help your body fight the virus. This can help lessen symptoms, decrease outbreaks, and lower the risk of passing the virus to others. Correct usage of latex condoms can reduce, but not eliminate, the risk of catching or spreading herpes.

Learning Activity

Watch this video about Molly's story and herpes. (View the transcript here.)

HPV (or genital warts)

Human papillomaviruses (HPV) are common viruses that can cause warts. There are more than 100 types of HPV. Most are harmless, but about 30 types put you at risk for cancer. These types affect the genitals and you get them through sexual contact with an infected partner. They are classified as either low-risk or high-risk. Low-risk HPV can cause genital warts. High-risk HPV can lead to cancers of the cervix, vulva, vagina, and anus in women. In men, it can lead to cancers of the anus and penis.

Although some people develop genital warts from HPV infection, others have no symptoms. Your health care provider can treat or remove the warts. In women, Pap smears can detect changes in the cervix that might lead to cancer.

Correct usage of latex condoms greatly reduces, but does not eliminate, the risk of catching or spreading HPV. A vaccine can protect against several types of HPV, including some that can cause cancer.

Trichomoniasis

Trichomoniasis is a sexually transmitted disease caused by a parasite. It affects both women and men, but symptoms are more common in women. Symptoms in women include a green or yellow discharge from the vagina, itching in or near the vagina and discomfort with urination. Most men with trichomoniasis don't have any symptoms, but it can cause irritation inside the penis.

You can cure trichomoniasis with antibiotics. In men, the infection usually goes away on its own without causing symptoms. But an infected man can continue to infect or reinfect a woman until he gets treated. So it's important that both partners get treated at the same time. Correct usage of latex condoms greatly reduces, but does not eliminate, the risk of catching or spreading trichomoniasis.

AIDS and HIV

AIDS was first reported in the United States in 1981 and has since become a major worldwide epidemic. AIDS is caused by the human immunodeficiency virus, or HIV. By killing or damaging cells of the body's immune system, HIV progressively destroys the body's ability to fight infections and certain cancers. People diagnosed with AIDS may get life-threatening diseases called opportunistic infections. These infections are caused by microbes such as viruses or bacteria that usually do not make healthy people sick.

Since 1981, more than 980,000 cases of AIDS have been reported in the United States to the Centers for Disease Control and Prevention (CDC). According to CDC, more than 1,000,000 Americans may be infected with HIV, one-quarter of whom are unaware of their infection. The epidemic is growing most rapidly among minority populations and is a leading killer of African-American males ages 25 to 44. According to CDC, AIDS affects nearly seven times more African Americans and three times more Hispanics than whites. In recent years, an increasing number of African-American women and children are being affected by HIV/AIDS.

  • The U.S. HIV/AIDS epidemic began in 1981 and continues to disproportionately affect minorities, men who have sex with men of all races, women, and youth.
  • More than 1 million people in the United States currently are living with HIV/AIDS.
  • About 21 percent of those infected with HIV are unaware of their infection.
  • Since the U.S. epidemic began, 617,025 people have died of AIDS.
  • In 2008, there were approximately 42,439 new HIV infections, with the highest proportion among African Americans despite the fact that they make up only 12 percent of the U.S. population.


HIV

HIV stands for human immunodeficiency virus. It kills or damages the body's immune system cells. AIDS stands for acquired immunodeficiency syndrome. It is the most advanced stage of infection with HIV.

HIV most often spreads through unprotected sex with an infected person. It may also spread by sharing drug needles or through contact with the blood of an infected person. Women can give it to their babies during pregnancy or childbirth.

The first signs of HIV infection may be swollen glands and flu-like symptoms. These may come and go a month or two after infection. Severe symptoms may not appear until months or years later.

A blood test can tell if you have HIV infection. Your health care provider can perform the test.

There is no cure, but there are many medicines to fight both HIV infection and the infections and cancers that come with it. People can live with the disease for many years.

AIDS Prevention

Currently, there is no vaccine to prevent HIV infection nor is there a cure for HIV/AIDS. To reduce your risk of becoming infected with HIV or transmitting the virus to others

  • Get tested regularly for HIV
  • Practice abstinence
  • Remain faithful to your spouse or partner
  • Consistently use male latex or female polyurethane condoms
  • Do not share needles


Detection and Treatment in HIV Prevention

Testing and treatment of sexually transmitted diseases (STDs) can be an effective tool in preventing the spread of HIV, the virus that causes AIDS. An understanding of the relationship between STDs and HIV infection can help in the development of effective HIV prevention programs for persons with high-risk sexual behaviors.

What is the link between STDs and HIV infection?

Individuals who are infected with STDs are at least two to five times more likely than uninfected individuals to acquire HIV infection if they are exposed to the virus through sexual contact. In addition, if an HIV-infected individual is also infected with another STD, that person is more likely to transmit HIV through sexual contact than other HIV-infected persons (Wasserheit, 1992).

There is substantial biological evidence demonstrating that the presence of other STDs increases the likelihood of both transmitting and acquiring HIV.

  • Increased susceptibility. STDs appear to increase susceptibility to HIV infection by two mechanisms. Genital ulcers (e.g., syphilis, herpes, or chancroid) result in breaks in the genital tract lining or skin. These breaks create a portal of entry for HIV. Additionally, inflammation resulting from genital ulcers or non-ulcerative STDs (e.g., chlamydia, gonorrhea, and trichomoniasis) increase the concentration of cells in genital secretions that can serve as targets for HIV (e.g., CD4+ cells).
  • Increased infectiousness. STDs also appear to increase the risk of an HIV-infected person transmitting the virus to his or her sex partners. Studies have shown that HIV-infected individuals who are also infected with other STDs are particularly likely to shed HIV in their genital secretions. For example, men who are infected with both gonorrhea and HIV are more than twice as likely to have HIV in their genital secretions than are those who are infected only with HIV. Moreover, the median concentration of HIV in semen is as much as 10 times higher in men who are infected with both gonorrhea and HIV than in men infected only with HIV. The higher the concentration of HIV in semen or genital fluids, the more likely it is that HIV will be transmitted to a sex partner.


How can STD treatment slow the spread of HIV infection?

Evidence from intervention studies indicates that detecting and treating STDs may reduce HIV transmission.

  • STD treatment reduces an individual's ability to transmit HIV. Studies have shown that treating STDs in HIV-infected individuals decreases both the amount of HIV in genital secretions and how frequently HIV is found in those secretions (Fleming, Wasserheit, 1999).
  • Herpes can make people more susceptible to HIV infection, and it can make HIV-infected individuals more infectious. It is critical that all individuals, especially those with herpes, know whether they are infected with HIV and, if uninfected with HIV, take measures to protect themselves from infection with HIV.
  • Among individuals with both herpes and HIV, trials are underway studying if treatment of the genital herpes helps prevent HIV transmission to partners.


Learning Activity

Watch this video about AIDS from PBS.

Illness, Sickness, and Disease

Discussing the complexities of what constitutes a disease requires careful distinction among related, but distinct concepts. In 1973, Susser, an epidemiologist, proposed some definitions that remain useful. He used "illness" to refer to the subjective sense of feeling unwell; illness does not define a specific pathology, but refers to a person’s subjective experience of it, such as discomfort, tiredness, or general malaise. The way a patient reports symptoms is influenced by his or her cultural background, and Susser applied the term "sickness" to refer to socially and culturally held conceptions of health conditions (e.g., the dread of cancer or the stigma of mental illness), which in turn influence how the patient reacts ). The social perceptions of disease that Illich described modify the ways a patient perceives and presents his symptoms.

Cultural conventions likewise affect where the boundary between disease and non-disease is placed: menopause may be considered a health issue in North America, but symptoms are far less commonly reported in Japan. Disease implies a focus on pathological processes that may or may not produce symptoms and that result in a patient’s illness. For example, a patient complains of tiredness and malaise–his illness as he experiences it. He consults a doctor about it–because he believes that he might have a sickness. The doctor might attribute the patient’s symptoms to a thyroid condition–a disease.

This model focuses on pathological processes, and on understanding, diagnosing, and treating the physical and biological aspects of disease. The goal of treatment is to restore the patient’s physiological integrity and function. Diagnosis involves recognizing and applying a label to a pattern of signs and symptoms that is at least partly understood in terms of abnormal structure or function of cells, organs, and systems. This offers a rational basis for the investigation of effective treatments. For instance, a certain pattern of chest pain known as angina pectoris is understood biologically as a disorder of the coronary arteries that causes cardiac ischemia, and the treatments it are geared to the specific causes of restoring cardiac blood flow and reducing cardiac effort.

Early biomedical conceptions supposed that a disease is either present or absent: a bacterium has invaded the body or it has not. However, as medicine increasingly tackled conditions, such as hypertension, which represent deviations from normal values, which themselves have a range and can be debated, it became apparent that there may be no set threshold for defining disease. Thus, instead of being seen as a state that is qualitatively distinct from health, many diseases have to be approached as a quantitative threshold on a continuum of biological variability. Organizations such as the World Health Organization (WHO) and the National Institutes of Health have proposed different classifications of hypertension and have changed how they constitute hypertension over time. Hypertension can be mild, moderate or severe, or defined as pre-hypertension or hypertension stage 1 or stage 2.

Optional: Video Lectures about Epidemics

Epidemics in Western Society is a course taught Frank Snowden at Yale.

This course consists of video lectures with an international analysis of the impact of epidemic diseases on western society and culture from the bubonic plague to HIV/AIDS and the recent experience of SARS and swine flu.

Leading themes include:

  • infectious disease and its impact on society
  • the development of public health measures
  • the role of medical ethics
  • the genre of plague literature
  • the social reactions of mass hysteria and violence
  • the rise of the germ theory of disease
  • the development of tropical medicine
  • a comparison of the social, cultural, and historical impact of major infectious diseases
  • the issue of emerging and re-emerging diseases






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