When we think about the relationship between human beings and their environment, we usually end by pondering the profoundly negative effect that our species has had on so many other species, often to the point of driving them from the face of the earth. But many of our manipulations also have made it more difficult for our own species to survive, not only because we've poisoned the environment with sulfur dioxide, lead, and dioxin, but because our changes have allowed many disease organisms to become much more dangerous. By changing the environment, we have not only killed off many organisms, but have created ideal conditions for others, and many of these-black flies, mosquitoes, snails-are vectors for a host of parasitical diseases. No matter where in the United States we teach, in cities, suburbs, or rural areas, we are surrounded by enough environmental problems to keep our students occupied indefinitely. So why should we bother presenting the problems of parasitical diseases, particularly those caused by animal parasites-protozoa and worms? These are primarily tropical diseases. Our students worry about developing heart disease or cancer. They don't give a second thought to malaria or schistosomiasis, and, practically speaking, they don't have to. If they remain within the United States, their chances of contracting most of these diseases are relatively slim. But there cire good reasons for making students aware of these diseases. The immunologist Sir Macfarlane Burnet has said, "If we take as our standard of importance the greatest harm to the greatest number, then there is no question that malaria is the most important of all infectious diseases" (Natural History of Infectious Disease). As inhabitants of this planet, our students should have some appreciation for what their fellow inhabitants have to contend with. Acquainting them with malaria, schistosomiasis, or sleeping sickness helps to relieve their biological isolationism. Nor should we feel too superior to other peoples. We have our own problems with parasites. In 1912 it was estimated that there were a million cases of malaria a year in the southern United States, and that 40 percent of the population in these states were infected with hookworms (The Plague Killers by Greer Williams). Intensive eradication campaigns brought these diseases under control, but recent outbreaks of parasitical infections in this country are a constant reminder that, given an opportunity-that is, a suitable change in the environment-these organisms can rise up to plague us again. It's been estimated that from two to 20 percent of Americans are infected with the protozoan Giardia, an intestinal parasite that many travelers bring back to the U.S. Giardia, which causes persistent diarrhea, passes from its host into sewage. Outbreaks have occurred in several cities where the drinkMaura C. Flannery is Associate Professor of Biology at St. John's University, Jamaica, NY 11439. She earned a B.S. in biology from Marymount Manhattan College and an M.S., also in biology, from Boston College. Her major interest is in communicating science to the nonscientist. She has developed a biology course for criminal justice majors as well as courses in reproductive biology and in the future impact of biological research.