Microscopic waterborne organisms are replacing potential cancer-causing chemicals as the primary threat to drinking water in some U.S. cities, according to health officials.

"Most of these organisms have been around for eons, but we're just now beginning to detect them," said Dennis Juranek of the Centers for Disease Control and Prevention. He noted that more people are aware of the problem because of reports of serious illness and deaths caused by microbial complications.

The centers estimate that up to 1,000 people die annually and as many as a million are sickened from microbial illnesses stemming from drinking water. In 1993 some 100 people died in Milwaukee from water containing cryptosporidium, an organism resistant to chlorine, the disinfectant most effective and most widely used for drinking water in the United States.

Most healthy people are relatively unaffected by ingesting such bacteria, parasites and viruses and may feel discomfort that can be mistaken for intestinal viruses. However, for those with weakened immune systems unable to fight off bacterial attack-such as cancer patients, the elderly and those suffering from HIV-related infections-water containing such organisms can be fatal, as was shown in Milwaukee.

Even in Washington, D.C., residents have expressed concerns about their drinking water since the city's aging water pipes were found to harbor large concentrations of bacteria.

Many of these organisms have only recently been detected, and efforts are under way to study them to find ways to prevent further threat to the nation's drinking-water supplies. (Source: The Associated Press.)