| Infectious diseases are illnesses caused by the invasion and reproduction of foreign microorganisms, termed pathogens, in the human body. These pathogens may be bacterial, viral, parasitic, and even fungal. Pathogens may be derived from external sources, such as interpersonal transmission or foodborne contamination. Other pathogens already exist in the normal flora of the body, but immunocompromise or a related event may cause these otherwise innocuous pathogens to cause disease.
Some pathogens do not complete replication in one host. Certain parasites require an external animal host to complete its own life cycle. In addition, there is currently much debate on the correct classification of the prions. Viruses marginally fall under the umbrella of a "microorganism", but prions are simply proteins normally found in humans that have been improperly folded. |