Because of modern living conditions, parasites have become an unsuspected public health problem. As far back as 1976, a random nationwide study conducted by the Centers for Disease Control (CDC) in Atlanta revealed that one in every six people who were studied carried one or more parasites.
People can be carrying just a few parasites and not suffer disease symptoms. When their immune systems are challenged by some other illness or accident, however, the parasites may also start to cause problems.
The chance of infection is significant in certain locales or in particular population groups. People who travel abroad are at a higher risk as are children in day-care centers and those who care for them.
Parasites are defined in Webster's New World Dictionary as any plant or animal that lives on or in an organism of another species and derives sustenance or protection from it, without benefitting--and usually harming--the host. Parasites that live in humans range in size from visible worms, such as tapeworms or pinworms, to microscopic organisms, such as Giardia lamblia.
A number of seemingly unrelated factors occurring in the late 20th century appear to have contributed to an increased risk of parasitic infection among North Americans. The major factors are:
1. International travel--Increased travel to parts of Asia, Africa, and Latin America has boosted the risk of acquiring foreignborn parasites.
2. Municipal and rural water supplies--Giardia lambia, a microscopic organism, is the prime culprit in water-borne disease. Once a threat only to foreign travelers and backpackers, this organism now appears across the U.S. in both rural and urban water systems. (It is introduced to water supplies by animals via mountain streams and by human sewage entering public water supplies.)
3. Day-care use--Because Giardia and other microorganisms can be spread through careless diaper changing, day-care centers are a source of parasite transmission.
4. Influx of immigrant populations--Many immigrants are asymptomatic carriers of parasites that produce symptoms in North Americans. Non-English-speaking immigrants often find work in restaurant kitchens where inadequate hygiene may allow parasites to be transmitted to the food.
5. Return of military personnel from overseas--Over half a million U.S. troops returning from the war in Iraq in 1991 were told not to donate blood because of the likelihood that they were carriers of a parasitic disease called leishmaniasis, which is spread by desert sand flies.
6. Household pets--Pets are hosts to numerous parasites (harbored in their fur and saliva). Around 240 different infectious diseases are transmitted from animals to humans.
7. Exotic regional foods--Raw, rare, or undercooked meat and fish dishes, and even water-grown vegetables like watercress and lotus root, may harbor parasites.
8. Antibiotics and immune-suppressing drugs--The reduction in beneficial intestinal flora following the use of these drugs weakens the body's defenses against hidden invaders.
9. The sexual revolution--Parasites become another type of sexually transmitted disease.
