A 2-year-old Indiana boy who contracted a rare and life-threatening infection from his soldier father's smallpox vaccination is recovering, a hospital spokesman said.

Doctors have relied on some untested measures to save the boy's life, including skin grafts and an experimental drug that has never been used to treat a human patient, officials said. The boy's pox lesions left him with the equivalent of second-degree burns, requiring grafts to let the underlying skin heal.

"Everyone has been a little bit astonished that he has recovered as well as he has," hospital spokesman John Easton said Saturday. The boy should be should be upgraded to serious from critical condition soon, he added.

The boy has been in pediatric intensive care at the University of Chicago's Comer Children's Hospital for the past month with a virulent rash over 80 percent of his body. He developed the rash after coming in contact with his father, who had recently been vaccinated for smallpox before he was to be deployed overseas by the Army.

The boy is not suffering from smallpox, but from the related virus which is used to convey immunity to the much deadlier disease.

Health officials say there is no infection risk for the general population because the virus can be spread only through close physical contact.

The child suffered from eczema, which is a known risk factor for such an infection, doctors said.

The military resumed smallpox vaccinations in 2002 because of bioterrorism fears.