The mere presence of parasites in a host should not be considered evidence of disease. The harboring of a few hookworms, pinworms, or tapeworms in an otherwise healthy host, for instance, does not constitute disease. There are relatively few parasites that may be considered primary pathogens; the majority cause recognizable disease only when the host is more or less handicapped by poor nutrition or other debilitating conditions.
Usually the amount of parasitic disease in a community depends on the balance between conditions affecting resistance, particularly nutrition, and those affecting degree of exposure to infection. The hookworm burden of a community, for instance, may depend more on the adequacy of a diet with respect to protein, vitamins, iron, and calories than it does on factors affecting acquisition of hookworms, such as climatic conditions, soil pollution, nature of soil, and wearing of shoes.
The environmental conditions affecting parasitic disease may be profoundly changed by human activity, either for good or bad. Such operations as mass treatments or vaccinations, destruction of arthropod vectors, drainage, cleanliness, water purification, careful handling and treatment of food, and sanitary excreta disposal are balanced by irrigation and creation of borrow pits, rice fields, step wells, and the like, where vectors may breed; also by distribution of disease carriers by planes, trains, or ships, use of night soil (human feces) for fertilizer, feeding pork scraps to pigs, migration of labor or military forces, and other activities that favor the breeding or dispersion of parasites or their vectors.
Animals in nature seldom suffer severely from parasites, although the numbers of gregarious rodents may be checked by them. Confined animals, however, more easily acquire and transfer parasites. Parasitic disease is common in fish hatcheries, zoos, and mink and fox farms. Domestic animals, because of their concentration and confinement, tend to suffer from parasitic diseases much more than their wild relatives. In some cases, however, their disadvantages are more or less compensated for by application of insecticides, medication, vaccination, breeding of resistant stock, and sometimes even sanitation.