Three Bad Bugs Here are a few of the most serious waterborne pathogens: Cryptosporidium: In March and April 1993, an outbreak of cryptosporidiosis in Milwaukee caused serious gastrointestinal upset in an estimated 400,000 people. An investigation indicated the nasty bug had bypassed the filtration system of one of the local water treatment plants. It's a tough waterborne illness to kill, particularly for those whose immune system has been compromised. Escherichia coli: Most people know E. coli — which can cause diarrhea and other gastrointestinal illness — as a food-related bacteria. In fact, you're more likely to be infected with E. coli after eating an undercooked hamburger than from diving into your local pool. However, it is possible to get an E. coli infection while swimming. For example, an E. coli outbreak in Battle Ground Lake in Oregon in 1999 sickened nearly 30 swimmers. And in 1998, at least nine children were infected with a particularly nasty strain of E. coli while swimming at a water park in Georgia. E. coli normally makes its home in the intestines of warm-blooded animals, including humans. Swimmers can become infected if the water is contaminated with fecal matter, as reportedly was the case in the Georgia outbreak — the first known instance in which E. coli was transmitted in a chlorinated pool. However, the greatest threats of swimming-related E. coli contamination have been near lake water. Shigella: In one 1995 shigellosis outbreak at a Pennsylvania lake, 70 people were infected. In another outbreak, this one in 1991, 38 people, most of them children, came down with this gastrointestinal disease after swimming in an Oregon lake. Researchers believe the source was fecal contamination. Like E. coli, Shigella is a form of intestinal bacteria.

These small but potent bugs are rarely life-threatening, except to people with immune system disorders such as AIDS. Even if you don't fall into this category, exposure to these microorganisms can result in some unpleasant symptoms.

Most of these infections result in nausea, vomiting and diarrhea, but some waterborne illnesses can be much more serious.

All waterborne illnesses are transmitted through fecal matter — sometimes from wildlife or farm animals, but often from humans. Not a pretty thought, but increasingly a concern for the folks who run public swimming pools and water parks, with implications for any parent with a child still in diapers. A few pathogens in particular are linked to some serious outbreaks. (See "Three Bad Bugs.")

Swimming Safety More than 300 children younger than age 5 drown each year in residential swimming pools, and hospital emergency rooms treat more than 2,000 children younger than age 5 for submersion injuries, according to the Consumer Product Safety Commission. Here's how to safeguard your child: Never leave toddlers alone near a pool. Don't assume baby pools are safe. The American Red Cross reports that 15 percent of home drownings occur in wading pools, toilets, buckets and other small containers of water. Fence the area around the pool, whether the pool is in-ground or above ground, and lock the gates. Add an alarm system to your pool to notify you if someone enters the water when no one is there to supervise. Learn cardiopulmonary resuscitation — in cases that call for resuscitation, seconds count when it comes to preventing death or permanent injury.

There's probably no way to completely shield yourself from waterborne pathogens, but it is possible to minimize your exposure. Here's how: