Health authorities said Thursday that at least 100 children at a preschool in Ansan, Gyeonggi Province, have reported experiencing symptoms of E. coli infection since the first patient was discovered on June 18.

Kim Jae-seon, senior official at the city’s health center, said 31 people linked to the outbreak at the preschool have been hospitalized thus far, some of whom have been diagnosed with a type of kidney failure known as hemolytic uremic syndrome. Experts say about 5 to 10 percent of people with E. coli develop this potentially life-threatening condition.

She said the bacteria’s presence was confirmed in 42 preschoolers and a teacher as of Thursday morning, with 96 more awaiting test results. 147 tested negative.

“As transmission can take place through contact, we have asked those who may have had exposure to self-isolate,” she said.

Pediatrician Dr. Ha Il-soo of Seoul National University Hospital said while most recover from E. coli in about week or two, the few that come down with the syndrome have nearly 50 percent chance of getting a serious illness requiring interventions such as dialysis.

“E. coli infections occur more commonly during summer, and amongst children,” he said.

The Korea Centers for Disease Control and Prevention said in a press release that the preschool in question has been shut down until the end of this month, and it was investigating the source of contamination.

Typical symptoms of an E. coli infection include diarrhea or bloody stools, stomach cramps, vomiting and lightheadedness, according to the state health protection agency. Children under 5 years old, older adults and people with weakened immune systems are especially vulnerable to the infection.

The year’s first E. coli outbreak, detected mid-May, also involved a preschool in Jeju City, the agency said. A total of eight children and their family members became sick.

To prevent infection, the agency recommends that people wash their hands for over 30 seconds frequently, cook food at safe temperatures and avoid raw food, untreated water and unpasteurized milk.

By Kim Arin (arin@heraldcorp.com)