A 2-year-old boy and his mother contracted a rare, life-threatening infection from his soldier father's smallpox vaccination, according to a published report.

The boy developed a virulent rash over 80 percent of his body earlier this month after coming in contact with his father, who had been vaccinated before he was to be deployed overseas by the U.S. Army, the Chicago Tribune reported Saturday.

Doctors stressed that the boy was not suffering from smallpox, but from the related vaccinia virus which is used to convey immunity to the much deadlier disease. They said the boy, who seemed to be improving, may lose 20 percent of his outer skin layer.

The boy and his mother remained hospitalized in a specially ventilated room, though there is no infection risk for the general population because the vaccinia virus can be spread only through close physical contact. The family's name was not released at their request.

Doctors said the infection was a rare condition called eczema vaccinatum, which has not been reported since at least 1990, when the U.S. military ended a previous program of smallpox vaccination. Smallpox was declared eradicated in 1980.

The military began smallpox vaccinations again in 2002 because of bioterrorism fears.

Doctors said the child suffered from eczema, which is a known risk factor for vaccinia infection. People with eczema are warned not to have close physical contact with the recently vaccinated because the condition allows the virus to enter the skin, they said.

The U.S. Defense Department and federal, state and local health authorities have been in daily contact with the hospital.

Dr. Madelyn Kahana, the hospital's chief of pediatric intensive care, said the boy had been treated with a potent antiviral drug, as well as with an anti-vaccinia agent supplied by the U.S. Centers for Disease Control and Prevention and the experimental drug ST-246, which had been untried as a therapy in humans.