The Tarrant County Public Health Department is being inundated with calls today from people who believe they might have been infected with the waterborne parasite Cryptospordium at Burger’s Lake.

“Hundreds of calls are coming, but we don’t have definitive numbers,” said Vanassa Joseph, a spokeswoman for the department.

She said that any figures the department compiled would be outdated within 15 minutes. Staff from the Centers for Disease Control and Prevention and state health department are helping to screen the calls.

The calls were prompted after the health department announced eight confirmed cases of the contagious disease on Thursday and the voluntary closure of Burger’s Lake in Fort Worth.

Ms. Joseph said investigators still are trying to determine whether the popular private lake was the source of the infection but that so far, no other explanations for the outbreak have been found.

Ms. Joseph said the owner of Burger's Lake told the county that it has hundreds of customers on weekdays and thousands on the weekends.

The eight people confirmed with the infection visited Burger's Lake in late June and early July. Symptoms of the disease appear on average a week after exposure, according to the CDC's Web site.

The illness often results in stomach cramps, diarrhea, vomiting and dehydration, and it can sometimes be fatal.

Test results of water samples sent to the CDC should be available Saturday, Ms. Joseph said.

The 30-acre park has a 1-acre spring-fed pool, a water slide, a trapeze that swings over the water, beaches and picnic tables, according to its Web site. The outgoing voice mail message at the lake did not mention the outbreak of Cryptosporidium infections and only said the lake was temporarily closed and would "reopen upon further notice."

Daniel Fowler of Arlington was at Burger's Lake on July 5 for his daughter's 10th birthday party. About a week later, he said, nine girls and seven adults all became sick with symptoms similar to severe stomach flu.

"We thought that somebody had the flu and then it got passed around," Mr. Fowler said. "But looking at it, only the girls and the adults who got in the water got sick."

He said that staff at his doctor's office told him they had treated about 15 people with symptoms of cryptosporidiosis, and staff at Arlington Memorial Hospital told him they had seen about 20 people with those symptoms.

Anyone who visited Burger's Lake on June 21 or later or had symptoms of cryptosporidiosis is urged to call the county health department at 817-321-4700.

Cryptosporidium is one of the most frequent causes of waterborne diseases, according to the CDC. The parasite - commonly known as crypto - has a hard outer shell that makes it resistant to chlorine, according to a federal fact sheet. It can be found in both drinking water and recreational waters, such as swimming pools and lakes.

Dallas County Health and Human Services officials said they had 38 reported cases of cryptosporidiosis in 2006 and 38 more cases in 2007. Jacqueline Bell, a health department spokeswoman, said that those were isolated cases and that she wasn't aware of any large public outbreaks in recent years. Tarrant County figures were not available.

The disease can be particularly serious for people with weakened immune systems, including people with HIV, recent organ transplant patients and those being treated for cancer, according to the state health department.

Cryptosporidium is also the parasite that infected the Milwaukee water supply in 1993 and caused an estimated 403,000 people to become sick. The outbreak was believed to have contributed to more than 100 deaths, many of them people with weakened immune systems.

Burger's Lake - which is surrounded by the Naval Air State Joint Reserve Base Fort Worth - originally was used as a goldfish hatchery and aquarium in the 1920s. It later was transformed into a swimming hole and has been a popular site for church and school group gatherings and family reunions.