Clark County’s top public health official said Thursday that a child who recently tested positive for E. coli O157:H7 was in a play area at Lake Shore Athletic Club while possibly contagious.Dr. Alan Melnick said parents whose children visited the play area at the club in west Hazel Dell between July 10 and Aug. 4 should watch for symptoms.The club voluntarily closed the play area Thursday, Melnick said. The play area will be disinfected and remain closed until the health department determines there’s no risk for exposure.The child’s positive test for E. coli was reported to the health department.The original source of infection is not known, Melnick said.He said the child is doing OK. Melnick said the athletic club staff gave the health department attendance records of children who were in the play area, and the health department has been trying to contact more than 100 families.E. coli O157:H7 is a bacterial infection that can cause nausea, vomiting and diarrhea that may be bloody. Fever is usually mild or nonexistent.