All of the children appear to be suffering from hemolytic uremic syndrome - or HUS - a rare condition generally caused by infection with a virulent form of e. coli bacteria.
Investigators so far have found one link between the victims: All had attended the festival or the fair earlier this month. And all had contact with livestock at the events.
Health officials are taking a close look at a Plant City petting zoo that supplied animals to both attractions.
"These kids with HUS are the tip of the iceberg. There are other people who may have been infected but are not clinically ill," said Joann Schulte, an epidemiologist with the Florida Department of Health.
A public health alert in counties along the Interstate 75 and Interstate 4 corridors is intended to advise parents and health care providers to watch for signs of HUS.
"Normally what happens is the e. coli infection would cause diarrhea and cramps. Usually little or no fever is present and the illness is resolved in 5 to 10 days," Schulte said.
But in vulnerable populations - young children and the elderly - a toxin produced by the bacteria begins to destroy red blood cells and damage the kidneys, she said.
"There is a period between when somebody has the diarrhea and then several weeks later develops the HUS," Schulte said. "The toxin takes some time to do its work."
That's the critical message health officials are trying to disseminate.
"If your child has diarrhea - especially bloody diarrhea - looks pale or anemic, lethargic, has decreased urine output and has been to the county fair, they should call their doctor," said Mehul Dixit, a pediatric kidney specialist who is tending to the stricken children at Florida Hospital Orlando.
Three of the children at that hospital are in critical condition. A fourth is in guarded condition, according to hospital officials. All, ages 2 to 6, attended the Orlando fair.
The first arrived Sunday. A fifth child with suspected HUS was admitted Wednesday afternoon, Dixit said.
``We have one child on a ventilator and dialysis that we're extremely concerned about,'' Dixit said.
About 90 percent of HUS victims recover. Ten percent either die or have permanent kidney damage, he said.
"It's not uncommon to recover from HUS and years later show signs of kidney damage," said Dixit, who is medical director of Florida Children's Kidney Center. "The long-term is the unknown and the biggest worry."
Across town, a child who attended the Florida Strawberry Festival in Plant City was fighting for life at the Arnold Palmer Hospital for Children and Women in Orlando.
Hospital officials there would not release the child's age, gender or condition.
Orange County health officials say an unrelated adult at Arnold Palmer Hospital, who also attended the strawberry festival, may be a seventh case of HUS.
"They're in our hearts and prayers," said Patsy Brooks, general manager of the festival. "We're in the process of gathering information and working with the health department."
Meanwhile, a team of state and Orange and Hillsborough county health officials urgently sought to identify and contain the source of the infection.
"Both of these places had multiple points where people could be near animals," said Bill Toth of the Orange County Health Department. ``We have not yet ruled food out either."
The e. coli bacterium generally associated with HUS was responsible for a 1993 outbreak in Washington state that sickened several hundred people and led to the death of three children and one adult.
The source was undercooked ground beef from a Jack-in-the-Box restaurant.
Contaminated meat, unpasteurized milk and human sewage are common sources of the infection. The U.S. Centers for Disease Control and Prevention estimates 73,000 cases of infection resulting in 61 deaths each year can be attributed to the e. coli. About 2 percent to 7 percent of the cases lead to HUS complications.