A UC Davis medical ecologist is using epidemiological methods to track the source and transmission of disease-causing microbes that can travel from livestock to humans via lakes, rivers and streams. Dr. Rob Atwill, a veterinarian working out of the Veterinary Medical Teaching and Research Center in Tulare, Calif., studies infectious diseases such as that caused by the one-celled parasite Cryptosporidium. His research supplies practical grazing guidelines for livestock on lands adjacent to streams and reservoirs. "I'm trying to find solutions that will allow ranchers to continue working the land while protecting the public water supply," says Atwill. Cryptosporidium, which seems to infect nearly all mammalian species such as livestock and wildlife species, causes severe stomach cramps, diarrhea and fever when it infects humans. In 1993, 400,000 people in Milwaukee were sickened in an outbreak of the parasitic illness attributed to contaminated drinking water. Found in 60 to 90 percent of all surface waters, Cryptosporidium is unfazed by chlorination and can slip through many water filtration systems. The disease can be life-threatening for anyone with a weakened immune system, including people with AIDS.