Just as schools usher in a new group of students, plus all of their germs, hundreds of children in Denver have come down with an unusual and severe respiratory illness that has ailed communities across the U.S. in recent weeks.

Officials at Children’s Hospital Colorado told the Denver Post that the hospital has treated more than 900 children for the illness since Aug. 18. Similar outbreaks have been reported in geographic clusters around the Midwest this summer, including in St. Louis.

Health officials believe that the sickness is related to a rare virus called human enterovirus 68 (HEV68), the Post says. HEV68, first seen in California in 1962, and an unwelcome but highly infrequent visitor to communities worldwide since then, is a relative of the virus linked to the common cold (human rhinoviruses, or HRV), according to the Centers for Disease Control and Prevention.

HEV68, which almost uniquely affects children, tends to first cause cold-like symptoms, including body aches, sneezing and coughing. These mild complaints then worsen into life-threatening breathing problems that are all the more dangerous to children with asthma. Since viruses do not respond to antibiotics, hospitals have treated the illness with asthma therapies.

Although extremely unpleasant, no deaths have so far been reported from this summer’s outbreak.

There is no vaccine for HEV68, and health officials are encouraging the same practices that guard against the common cold: keep your hands to yourself, and wash them often.

Write to Elizabeth Barber at elizabeth.barber@timeasia.com.