At least four Americans have died of a severe respiratory illness that has spread to 42 states, public health officials said Wednesday.

The outbreak of enterovirus 68 has caused a rash of child illnesses nationwide, some of them critical. The Centers for Disease Control and Prevention has confirmed 500 cases as of Wednesday but said the number of cases is probably much higher.

The four patients who died were the first deaths in the U.S. linked to the virus, though the CDC said it’s not clear how much the virus contributed to their deaths. The only death described in detail was of a 10-year-old girl from Rhode Island, who died within 24 hours of being admitted to a hospital last week. Her parents brought her in because she had developed symptoms of a common cold and had difficulty breathing.

The infection usually causes fever, coughing and other symptoms similar to a bad cold. Children with asthma are particularly vulnerable to developing breathing problems that require emergency care. The first cases of enterovirus were reported in August at Children’s Mercy Hospital in Kansas City, Missouri.

Before the outbreak, enterovirus was linked to cases of sudden-onset paralysis in California. At Children’s Mercy Hospital, health officials treated about 850 children who exhibited symptoms of the virus, Dr. Mary Anne Jackson told the New York Times. Three of those children have polio-like symptoms, but they have not been diagnosed with enterovirus. Jackson said the cases of infection peaked early in September and are now falling slowly as the normal season for enterovirus comes to an end.