A. The Ubiquitous Organisms
Parasites are everywhere. There are far more kinds of parasitic than nonparasitic organisms in the world, and organisms that are not parasites are usually hosts, harboring parasites within and upon them. Their diversity in size, complexity, and life cycles is truly astonishing, ranging from single-celled amoebas responsible for diarrheal distress to the multicellular tapeworms that can grow to 20 feet in humans and 100 feet in whales. Table 1 illustrates examples of different types of parasites.
Table 1. Examples of parasites
B. The Parasitic Way of Life
Most parasites, however, are multicellular. Parasitic worms include tapeworms, which live in the digestive tract and cause anemia, and schistosomes, which inhabit the veins of the bladder or intestines and are the causative agents of the disease schistosomiasis. Nematodes, another form of worm, are responsible for heartworm in dogs and blindness in humans. Arthropods, such as fleas and ticks, are temporary parasites that visit the host for frequent or occasional feedings and can also act as vectors (carriers) of parasites that cause disease. Parasitic fungi, including mushrooms, molds, and mildews, feed on plants or animals. By this definition of parasite, certain bacteria and viruses can be considered to live as such, but conventionally the term parasite refers only to eukaryotic organisms—that is, organisms that have a nucleus and other subcellular organelles.
Parasites can be transmitted in a variety of ways: in contaminated food or drinking water, by swimming in lakes and rivers, or in the feces or saliva of an insect or other animal. Table 1 indicates modes of transmission for several parasites.
A parasite is often associated with damage to the host (see Table 1). A parasite may harm its host in any number of ways: by mechanical injury, such as boring a hole in it; by eating or digesting and absorbing its tissues; by poisoning the host with toxic metabolic products; or simply by robbing the host of necessary nutrients. Most parasites inflict a combination of these conditions on their hosts. Of course, the parasite is only trying to survive, taking from its environment—its host—what it needs to sustain its life processes so that it can reproduce. Parasites do not have evil intent, any more than bacteria or viruses do.
C. Prevention and Treatment
Although no effective vaccines for any human parasitic disease exist yet, good hygiene, clean drinking water, sanitary facilities, and effective programs of insect control can prevent many parasitic diseases. A simple change of habits, such as staying out of lakes and rivers or sleeping under mosquito netting, could prevent many serious, debilitating diseases.