Researchers are making headway in understanding the waterborne pathogen cryptosporidium, but many questions remain Was it a dead kangaroo in the reservoir? Bats in the pipes? Just a big sampling mistake? The jury is still out on the culprit responsible for last summer's cryptosporidium scare in Sydney, Australia. Even though no outbreaks of illness occurred, the city's three million residents were under a "boil water alert" for 3 months after elevated levels of the tiny pathogen were detected

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... a privately operated filtration plant. The incident embarrassed and infuriated city politicians. As host of the summer Olympics in 2000, Sydney's image was at stake. Although the scare has since been deemed a false alarm by most experts, it reminded public health officials and scientists alike that the potential for major cryptosporidiosis outbreaks still exists and that many important questions about this pathogen remain unanswered. That cryptosporidium can pose a serious health threat through a public water supply was made abundantly clear in April 1993, when 403,000 people in Milwaukee contracted cryptosporidiosis in the largest and most notorious waterborne outbreak of the disease to date. More than 100 people, most of them AIDS patients, subsequently died from complications of the parasitic disease. Victims of cryptosporidiosis suffer from voluminous watery diarrhea, occasionally accompanied by severe abdominal cramping, vomiting, low grade fever, and weight loss-symptoms that usually clear up within 1 or 2 weeks. But for people with impaired immune function, symptoms can intensify and linger because there is no effective treatment for the disease. The name cryptosporidium, meaning "hidden spore," is appropriate for this organism because, despite the intense efforts of researchers in the last two decades, many critical questions remain. Scientists have traced cryptosporidium's origins, mapped its life cycle, even recently identified two distinct genotypes that can infect humans. But the normal human immune response to an episode of the disease is not fully understood, nor is the pathogenesis of the organism itself. Moreover, the US Environmental Protection Agency (EPA) is unable to directly regulate the amount of cryptosporidium allowed in public water supplies because no methods can reliably detect the pathogen in water samples. Even if detected, cryptosporidium is difficult to eliminate with standard water treatment practices. The parasite is most threatening to patients in the late stages of AIDS, in whom cryptosporidial infections resist practically every drug tested by clinical researchers. Commonly misrepresented in the popular media as bacteria, the eight known species of cryptosporidium (often referred to simply as "crypto") are in fact protozoans, and they infect every vertebrate family. Humans' particular nemesis is Cryptosporidium parvum, first described in 1907 by Harvard biologist Ernest Tyzzer, who observed it in mice. Outside the host, all cryptosporidium species exist as oocysts-spores encased in tiny resilient round capsules. Parvum means "small" in Latin, and at only 3-7 micrometers in diameter, approximately one-twentieth the thickness of a human hair, C. parvum