What Causes Communicable Diseases?

Communicable diseasesare the result of three factors: the agent, the environment, and the host. The agent is the original cause of the sickness. Disease agents include bacteria (causing such maladies as typhoid fever and cholera), protozoa (resulting in diseases like amoebic dysentery), viruses (causing polio, infectious hepatitis, and so forth), parasites (causing malaria, and so forth), and fungi (responsible for problems like athlete's foot). There are also nonmicrobic agents like lead and mercury, which can cause poisoning.

The disease agent exists in what are called reservoirs. These may be an already sick person, a carrier (someone who carries the agent, but who has no symptoms of the disease), an animal, or even a part of the inanimate environment. When the agent is expelled from the reservoir—by coughing, sneezing or in some other way—it may be picked up and transported to a potential host, that is, someone who is susceptible to catching the disease. If the agent finds the right way into the host, illness will result. The importance of the way that the agent enters is seen in the case of tetanus. If the germ enters through the mouth, it is harmless. However, if it gets in through a deep cut in the skin, the host probably will become sick with the disease.