The number of Onondaga County residents sickened by a gastrointestinal illness that surfaced in late June has climbed to 55 and public health officials still have not identified the source of the outbreak.

Children under 10 account for about half of the 55 confirmed and probable cases of shigellosis, a bacterial infection, said Dr. Cynthia Morrow, Onondaga County’s health commissioner.

Many people who are infected develop fever, painful blood or mucous diarrhea, fever and stomach cramps a day or two after being exposed to the bacteria, called shigella. The illness usually clears up in five to seven days. Severe cases need to be treated with antibiotics.

A few of the people infected have been hospitalized, Morrow said.

The bacteria is present in the stools of infected people. It can be spread when infected people fail to wash their hands before handling food eaten by others or coming into contact with others.