In the first case of its kind in years, a 2-year-old boy is being treated in Chicago for a rare and life-threatening infection that he contracted from his father, a U.S. Army soldier recently vaccinated against smallpox.

The Indiana boy is in critical condition with eczema vaccinatum, an unusual side effect of the smallpox vaccine that can affect people who receive the shot or their close contacts.

Doctors also said the boy appears to have passed the infection to his mother, who has a much milder case of the virus in the smallpox vaccine, which is also called vaccinia. The virus is not smallpox, though it is similar enough to offer protection from that deadly disease, which was declared eradicated in 1980.

The mother and child are being treated at the University of Chicago's Comer Children's Hospital, which withheld their names at the family's request. There is no infection risk for the general population, government officials say, since the vaccine virus can spread only through close physical contact.

But the boy's diagnosis last week has prompted a frenzy of activity and daily conference calls involving the federal Centers for Disease Control and Prevention, the U.S. Department of Defense, and the state and city public health departments. The U.S. Food and Drug Administration gave emergency authorization for the hospital to treat the boy with ST-246, an experimental drug for smallpox that is untried as a therapy in humans.

The smallpox vaccine fell out of general use in the 1970s, but the case could be a lesson for the U.S. military, which has vaccinated 1.2 million personnel against smallpox since 2002 amid fears of bioterrorism.

It's unclear why the father was allowed to have contact with his son, who had a history of eczema, shortly after the vaccination. The skin condition is a well-known risk factor for eczema vaccinatum, and official guidelines warn that people with eczema should avoid contact with vaccinees.

"We are looking into how this could have happened," said U.S. Army spokesman Paul Boyce.

Officials say the general population could receive smallpox vaccinations in the event of a bioterrorist attack or other unforeseen exposure. For that reason, experts want to study the Indiana family to learn more about treatment and transmission of the vaccinia infection.

"There certainly are also conceivable insights into smallpox infection," said Dr. Inger Damon, chief of the CDC's poxvirus and rabies branch. Damon has been involved in the daily conference calls on the boy's treatment.

Experts said they knew of no cases of eczema vaccinatum since at least 1990, when the military last had a program of smallpox vaccination.

The vaccinia virus in modern smallpox vaccines is closely related to an older form of vaccinia called cowpox, the disease English doctor Edward Jenner used in the late 1700s to develop early methods of vaccination.

Jenner relied on the observation that milkmaids who had cowpox seemed to be protected from later smallpox infection. He found that patients inoculated with material from cowpox sores also got protection from smallpox. That history is why the word vaccine stems from the Latin word for cow.

Vaccinia was modified from its original form over the years but remains an infectious agent with the potential for side effects.

The father of the Indiana boy received the vaccine in late January before a planned military deployment. The Army delayed his departure and permitted him to visit his family in mid-February.

Two weeks later, a rash broke out on the boy's skin. He came to the U. of C. on March 3 after being transferred from St. Catherine's Hospital in East Chicago. Doctors first identified his widespread rash as a different form of eczema, but it worsened in his first few days at the U. of C.

His mother developed sores after she and her son arrived at the Chicago hospital. Doctors believe she contracted the disease from the boy because of their lengthy close contact.

A pediatric dermatologist, Dr. Sarah Stein, noticed the boy's lesions had changed to look like round blisters with a dimple in the middle--a potential sign of vaccinia infection. The medical team took scrapings from the lesions, which they analyzed and sent to the Illinois Department of Public Health's Chicago office for further testing.

Rapid tests by the state and further tests at the CDC confirmed the boy had the vaccinia virus, officials at those agencies said. The hospital also sent the CDC photos of the boy's lesions.

The hospital already was using infection precautions with the boy, but staffers then added such measures as gloves and face masks. They also placed the boy in a room with negative pressure so the air would always blow inward, keeping the virus inside.

The boy's rash had spread to cover 80 percent of his body, said Dr. Madelyn Kahana, chief of pediatric intensive care medicine at the U. of C. He was going into sepsis, a devastating, systemwide infection rarely seen with viral cases.

"In the later stages of [eczema vaccinatum], it can look like smallpox," said Damon of the CDC.

The boy needed a ventilator to help his breathing because of the powerful pain medication he needed for the lesions.

The boy received the primary treatment for eczema vaccinatum, a drug called vaccinia immune globulin, or VIG. The drug came from a stockpile the CDC keeps in case widespread vaccination ever becomes necessary.

He also got an antiviral drug called cidofovir and the experimental drug ST-246, which has been shown to protect laboratory animals from exposure to smallpox. The drug recently entered preliminary human trials but had never been used in a sick patient.

U. of C. officials said the boy has shown signs of improvement since hitting a low point last weekend. His mother's health was never in serious danger, but she has remained in his hospital room to keep others from being exposed. Health officials in Chicago and Indiana have tracked all of the family's contacts and found no additional cases so far.

Kahana said the boy probably will lose 20 percent of his outer skin layer, but she hopes he will recover without the need for skin grafts. She believes the case should be a lesson to the military, which must educate service members about the risks of the vaccines it requires them to take.

"I think the information simply wasn't disseminated properly or impressed in a manner that was understood," Kahana said, "because I don't think anyone would knowingly expose their child to this."