A sickly child, Melissa was plagued with chronic infections, so her mom decided to let her skip her smallpox vaccination.

Doctors didn't figure out until Melissa was 11 that she lacked a natural supply of three infection-fighting antibodies, which put her at risk of a massive infection -- from the live virus, called vaccinia, used to make smallpox vaccine.

''Mothers' intuition,'' says Schweitzer, now 34, who gets monthly antibody infusions to protect her from ordinary infections. ''There's something to be said for it.''

Schweitzer, of Annapolis, Md. is one of about 60 million people in the USA with conditions that leave them essentially defenseless against vaccinia.

They include people with HIV (news - web sites)/AIDS (news - web sites) or other immune deficiencies, people who have had organ transplants, cancer patients undergoing chemotherapy and people with eczema and certain other skin diseases.

Health authorities also advise against vaccinating women who are pregnant or who might become pregnant because no one knows how live vaccinia might affect fetuses.

Vaccinia is a much weaker version of its relatives, smallpox and cowpox. In most cases, it is readily conquered by a healthy person's immune system. But even in healthy people, vaccinia can have life-threatening side-affects.

Research done in the 1960s found that 12 people per million develop an infection of the brain called encephalitis. One or two per million die. The research suggests that widespread vaccinations would cause about 4,600 serious side effects and 285 deaths.

Anyone whose condition hasn't been diagnosed, like young Melissa, faces the greatest risks. That population includes about 300,000 people in the USA with undiagnosed HIV. Some may become infected simply by coming in contact with someone who has just been vaccinated with smallpox vaccine.

The Bush vaccination plan, to be released today, represents a relatively small-scale beginning to mass vaccinations. But even a limited program could unleash vaccinia among some people who are defenseless against it.

Because the risk of a smallpox attack is unknown, health authorities can't accurately balance the risks of vaccination against the likelihood of a smallpox outbreak. Until a case appears, the risk of the vaccine will always be greater than the risk of getting smallpox. And any vaccine-related illnesses could have a far-reaching impact on public acceptance of vaccines.

''Experts are worried,'' says Tara O'Toole, of the Center for Civilian Biodefense at Johns Hopkins University's Bloomberg School of Public Health, ''that a general vaccination program will inevitably cause some lethal side effects . . . (that will) demolish the public's faith in vaccination and in following the government's bioterrorism recommendations.''

Once smallpox is released, the scenario changes dramatically because the disease is deadly, and it spreads rapidly. Smallpox kills about 30% of its victims, each of whom may pass the virus to as many as six people.

Everyone will be vaccinated if a case is detected. People who have had contact with a known or suspected case and who have not been vaccinated would be isolated to keep them from passing on the disease, says Marcella Layton, director of communicable diseases for the New York City Department of Health.

''Our medical advisers favor isolation over vaccination,'' says Jonathan Goldsmith, medical director of the Immune Deficiency Foundation, an advocacy group. ''We think that's your best choice. Keep kids out of school and parents out of work for a few weeks.''

O'Toole says she believes people will demand vaccination, ''especially when they see the first cases of smallpox on TV.''

Doctors can treat severe vaccine reactions with potent anti-vaccinia antibodies drawn from the blood of people who have gotten the vaccine. But that preparation, called vaccinia immune globulin, or VIG, is in short supply. Only 600 doses exist, but the government has ordered about 30,000 more.

An additional option is the drug Cidofivir, which is now licensed to treat retina infections in people with HIV and AIDS.

Doctors say Cidofivir could be used alone or along with VIG, making smallpox the latest in a small group of viruses that are beginning to yield to treatment.