RELATIONSHIP OF ENVIRONMENT TO INFECTION
The ecology of infection is complex and involves interactions with climate, food and water supply, arthropod vectors, animal contacts, and other human beings. Many of the great scourges of humankind, such as tuberculosis, cholera, malaria, and typhoid fever, were markedly decreased in incidence by changes in the environment. These changes anteceded development of effective vaccines and therapeutic agents. The greatest danger to Homo sapiens, regarding the spread of infection, are other Homo sapiens.
Organisms may spread from one person to another by direct contact, by the airborne route, or by oral ingestion of contaminated food or water. Food and water supplies may become contaminated with microbes derived from humans or animals or from the environment. For example, unpasteurized milk may serve to transmit disease if it is contaminated with organisms from an infected cow (for example, brucellosis) or from an infected dairy person (for example, streptococci emanating from a skin lesion). In some instances the microbe may actually grow and multiply on the food.
Density of population directly affects the spread of certain communicable diseases. Major epidemics have occurred in boarding schools and military barrack populations where the environment yields close contact among members. When people are confined to enclosed places, airborne pathogens tend to spread more readily.
Thus, the peaks of respiratory diseases such as influenza and pneumonia are the fall, winter, and early spring months in temperate climates. Patients interacting with animals may have special problems. For example, butchers and meat packers have the highest incidence of brucellosis, and hunters may contract tularemia through handling of rabbit pelts.
Insects are significant in the transmission of many infections. The malaria parasite is transmitted from one human to another, or from an animal to a human being, by the bite of the anopheles mosquito. Because this mosquito cannot survive in cold climates, malaria has never been a problem in countries where the climate is temperate. The tick vector of Rocky Mountain spotted fever requires bushy underbrush and a small mammal population to feed on for survival. Areas that do not have these conditions will not support the tick and are thus free of Rocky Mountain spotted fever. Schistosomiasis is a parasitic disease caused by an organism that spends part of its life cycle in a snail. If the snail does not have the proper water conditions to survive, the disease will not be found.