Diseases can be categorized as infectious or noninfectious. An infection is the invasion and multiplication of pathogens, or disease-causing organisms, within the host body. An infectious disease is a disease resulting from infection. In contrast, a noninfectious disease is a disease not caused by an infectious agent or pathogen. Potential causes of noninfectious disease include genetics, environmental contaminants, poor nutrition, and a combination of these factors. Noninfectious diseases are not spread from host to host (they are not contagious). Most cancers, heart disease, diabetes, Down syndrome, and multiple sclerosis are examples of noninfectious diseases. (However, some infectious agents have the potential to cause cancer. One example of this is cervical cancer. Some forms of cervical cancer may result from human papillomavirus infection.) Today, more deaths result from noninfectious diseases than from infectious diseases.
Infectious agents that cause disease are primarily viral, for example, adenovirus and hepatitis B virus, and bacterial pathogens, such as Streptococcus pyogenes and Pseudomonas aeruginosa. However, fungi, such as the opportunistic pathogen Candida albicans, and protozoans, such as Giardia lamblia, are also capable of causing disease. Other parasites also fall under this umbrella, for example, tapeworms. When ingested, the tapeworms take up residence in human intestines, where they parasitize their host. The protozoan Acanthamoeba is another parasite found ubiquitously in the environment in water and soil. It can cause severe corneal infections in people who do not clean and sanitize their contacts. Pathogens may come from the host's own microbial flora, another infected host, an insect vector, or any number of other routes.
Infectious diseases are categorized as communicable or noncommunicable. Communicable diseases are spread from host to host. A noncommunicable disease is an infectious disease that is not spread from host to host. Noncommunicable diseases are less common and typically arise from resident commensal microbes opportunistically infecting their host. Other possible routes to noncommunicable infection involve acquiring the infectious agent from its natural habitat. One example of this includes infection with Clostridium tetani, the causative agent of tetanus, from soil contamination through an open wound. Gardeners are at risk of infection from the fungus Sporothrix (also called rose gardener's disease), which can be acquired through small cuts in the skin or via inhalation. Other ways by which diseases can be obtained include through nosocomial and zoonotic sources. Nosocomial diseases are those that are passed on because an infection or toxin exists in a certain location, such as becoming sick after visiting someone in the hospital. A zoonotic disease is one that can be transmitted between animals and people, such as the bird flu or Trichinella, which is a parasite humans can get from eating undercooked pork.