At least six babies in Pennsylvania have been born infected with measles, and two were so sick they needed intensive care, as the state’s outbreak keeps growing.

Congenital measles, passed from mother to baby in the womb or during birth, is a condition doctors in the U.S. have rarely if ever seen. Measles was declared eliminated in America in 2000, and widespread vaccination long protected newborns.

“We’re literally seeing new forms of measles,” said Dr. John Goldman, an infectious disease specialist at the University of Pittsburgh Medical Center. “We don’t have a lot of information about what happens with it.”

Doctors can offer only supportive care: fluids, fever control and ventilation if needed. None of the doctors treating these babies have experience with the condition, Goldman said.

The outbreak has now reached 890 cases across 39 Pennsylvania counties, with 173 hospitalizations and four deaths. A possible measles exposure was also reported in the neonatal intensive care unit at Children’s Hospital of Philadelphia last week.

Health officials say declining vaccination rates and fewer young adults with natural immunity are behind the comeback of a disease the country once wiped out.