SOURCES AND COMMUNICABILITY
Infectious diseases of humans may be caused by exclusively human pathogens, such as Shigella; by environmental organisms, such as Legionella pneumophila; or by organismsthat have their primary reservoir in animals, such as Salmonella.
Noncommunicable infections are those that are not transmitted from human to hu-man and include (1) infections derived from the patient’s normal flora, such as peritonitis after rupture of the appendix; (2) infections caused by the ingestion of preformed toxins, such as botulism; and (3) infections caused by certain organisms found in the environ-ment, such as clostridial gas gangrene. Some zoonotic infections (diseases transmitted from animals to humans) such as rabies and brucellosis are not transmitted between hu-mans, but others such as plague may be at certain stages. Noncommunicable infections may still occur as common-source outbreaks, such as food poisoning from an enterotoxin-producing Staphylococcus aureus–contaminated chicken salad or multiple cases of pneumonia from extensive dissemination of Legionella through an air-condition-ing system. Because these diseases are not transmissible to others, they do not lead to secondary spread.
Communicable infections require that an organism be able to leave the body in aform that is directly infectious or is able to become so after development in a suitable en-vironment. The respiratory spread of the influenza virus is an example of direct commu-nicability. In contrast, the malarial parasite requires a developmental cycle in a biting mosquito before it can infect another human. Communicable infections can be endemic, which implies that the disease is present at a low but fairly constant level, or epidemic, which involves a level of infection above that usually found in a community or popula-tion. Communicable infections that are widespread in a region, sometimes worldwide, and have high attack rates are termed pandemic.