Health officials in Tampa say adults are probably unknowingly responsible for an outbreak of whooping cough.

Since Jan. 1, there have been 35 cases of pertussis -- also called whooping cough -- reported in Hillsborough County. The Florida Department of Health has reported 112 cases in Florida this year.

The Tampa Tribune reports the Hillsborough cases came from seven families.

A Hillsborough County Health Department spokesman says in one case an adult infected six children who did not have vaccinations.

Officials say the illness starts with a low grade fever, runny nose, sneezing and a cough. Antibiotics can treat the illness, but children often cough for six to eight weeks.

Health experts recommend booster shots for adults who expect to be around young children.