Denver ? Swimming pools are giving people diarrhea and putting more Americans at risk of contracting the illness, according to two yet-to-be-released federal studies.

The number of U.S. swimming-related outbreaks of illness caused by the parasite Cryptosporidium increased tenfold from 1990 to 2000, the Centers for Disease Control and Prevention says.

Americans who swim in pools are 10.6 times more likely to contract the parasite than those who do not, according to preliminary results from another CDC study. While neither study has been published yet, epidemiologist Michael Beach let scientists peek at the data Wednesday at the American Society of Tropical Medicine and Hygiene convention.

Over the past decade, the microscopic Crypto parasite has become resistant to chlorine, which is the main pool-cleansing agent, Beach said. The parasite, which lives in human and animal intestines, causes a disease also called Crypto, which brings on diarrhea, can include fever and lasts about two weeks. For people with weakened immune systems, it can lead to more serious disease.

Crypto is one of the major waterborne diseases in the United States, according to the CDC. And in the past two years more than 80 percent of the disease outbreaks from swimming pools have been due to the chlorine-resistant strain, Beach said.

The parasite is spread through feces. The biggest culprits behind its presence in swimming pools are children who are not yet potty-trained.

But pool contamination is due to more than just diapers. Most people don’t cleanse themselves sufficiently after bowel movements, which can add up to 2 to 3 pounds of feces a day in the average water park, Beach said.

The latest statistics on the disease, for 1999 and 2000, will be published next week; they show “a very sharp increase over the past two years” in the number of Crypto outbreaks from recreational swimming, Beach said.

Because 18 percent of the people who are ill with diarrhea continue to swim, the outbreak often resurfaces soon after swimming pools are drained and refilled, Beach said.

“It’s very hard to stop an outbreak like this,” he said. “The people won’t stop swimming.”

Preliminary results of a risk analysis of swimming show that people who use kiddie pools have a 10.7 times higher risk of contracting Crypto than those who do not swim. The risk is 10.6 times higher for swimmers in regular pools, 2.4 times higher for ocean swimmers and 1.7 times higher for lake swimmers, Beach said.