Epidemiology

Level of Disease Occurrence

Sporadic diseases occur only occasionally or in scattered incidents, whereas in an epidemic, there are larger-than-expected numbers of cases occurring over a short time in a geographic region. A pandemic is an outbreak that occurs in a large percentage of the world’s population over a vast geographic area.

In the study of epidemiology, disease is further classified based on how many people it affects and whether or not the disease represents a new outbreak or maintains a constant and stable presence in a geographic location. The following terms are used to classify disease: endemic, epidemic, holoendemic, hyperendemic and sporadic.

Endemic disease refers to the presence of a particular disease or pathogen that exists permanently and at a constant level in a population of people located in a specific geographic location. For example, in certain parts of Africa, malaria (caused by the Plasmodium protozoan species) is a constant concern.

Epidemic disease is a disease outbreak that occurs as a larger-than-expected number of cases occurring over a short time in a geographic region Influenza (flu) is a good example of an epidemic disease. Influenza can be caused by many strains of influenza viruses. It is a contagious respiratory illness that can lead to severe complications, including death in high-risk populations such as children, the elderly, or immunocompromised patients.

Holoendemic disease is a disease having high prevalence in early childhood and infecting most of a population’s children, resulting in a population in which children more often show evidence of the disease than adults. For many communities, malaria is an example of a holoendemic disease.

Hyperendemic disease is disease that is present at a high and constant frequency in a population and equally affects all groups within that population. Some examples in the U.S. population are hypertension, or high blood pressure, and osteoarthritis.

Sporadic disease is a disease that occurs only occasionally or in scattered incidents. Typhoid fever, also called enteric fever, is an example of a sporadic disease. Typhoid fever is caused by the bacterium Salmonella typhi and is life threatening. In the United States, approximately 700 cases per year occur. It is still common in less economically developed countries, where approximately 27.5 million cases occur every year.
The gram-negative bacillus Salmonella typhi is the causative agent of typhoid fever, or enteric fever. It is contracted via the fecal-oral route from contaminated water sources as well as from shellfish consumption.
Credit: NIAIDLicense: CC BY
A pandemic disease is disease outbreak affecting a large percentage of the world’s population over a vast (even worldwide) geographic region or regions. A historical example of pandemic disease includes the Black Death, also known as Black Plague, which resulted in the death of an estimated 75 to 200 million people in Europe and Asia in 1347 to 1351. The causative agent was Yersinia pestis transmitted via the bite of the rat flea. The 1918 influenza pandemic resulted in a widespread global infection of over 500 million people, 50 million of which died as a result. Another example of pandemic disease is Acquired Immune Deficiency Syndrome, or AIDS. AIDS is caused by the human immunodeficiency virus (HIV), which acts by destroying the CD4 helper cells of the immune system. CD4 helper cells are a subset of T cells, which are lymphocytes (white blood cells) that play a central role in cell-mediated immunity. The destruction of the CD4 cells makes the person afflicted with HIV more susceptible to infections. Certain factors predispose HIV/AIDS to becoming pandemic. These include a lack of sex/contraceptive education, unsafe blood transfusions, and IV drug use.

Epidemic diseases are tracked by the World Health Organization (WHO). A specialized agency of the United Nations, the WHO provides leadership on health matters, creates ethical and evidence-based health policy recommendations, assesses health trends, and monitors vital statistics, in an effort to eradicate epidemic, endemic, and other types of diseases. The WHO works with local governments to form epidemiologic and statistics services that monitor the location and spread of disease, the results of a treatment intervention, and tracks the progress of an intervention over time. Due to this type of data gathering, the WHO is able to identify impending public health emergencies. For example, the WHO has committed itself to malaria control. It works to achieve this by supporting funding for research to produce a viable malaria vaccine. It encourages the use of insecticide-treated mosquito netting and sprays to prevent malaria spread in Africa, as well as the use of antimalarial drugs, particularly for young children and pregnant women.