A two-year-old boy infected by his father’s smallpox vaccine is currently being treated at the University of Chicago Hospitals (UCH) in the first such case in over 30 years.

The boy is suffering from eczema vaccinatum, a rare skin disease caused by the smallpox vaccine that his father, a member of the armed forces, received in preparation for deployment.

When the father’s deployment was delayed, he returned home for several weeks, and it is believed that the close contact with his family infected the boy and his mother, who has experienced much milder symptoms.

The boy was transferred to UCH’s Comer Children’s Hospital in early March, where he remains on a ventilator in a sealed room with his mother.

“The child is still in the pediatric intensive care unit and still on the ventilator but is steadily improving and has come a long way,” said John Easton, a spokesman for UCH.

He has been treated with vaccinia immune globulin, the antiviral drug cidofovir, and the experimental antiviral treatment ST-246. The rash has spread to at least 80 percent of his body and is severe enough that it may require skin grafts over 20 percent, according to Easton.

Although routine smallpox vaccinations in the U.S. ended in 1972 and the World Health Organization declared the virus eradicated in 1980, military and emergency medical personnel began to be immunized again after September 11th.

The vaccine is based on the vaccinia virus, which is similar to, but separate from smallpox. Although adverse reactions to the vaccine are common—according to the Centers for Disease Control, one-third of people vaccinated are forced to miss work or school after being vaccinated—this is the first known case of a vaccinia infection spreading to an unvaccinated person since routine vaccination ended.