Smallpox may be the worst disease ever known to man. It killed about half a billion people from 1880 to 1980, before it was eradicated.

And the smallpox vaccine is deadly, too. Scientists call it the most dangerous vaccine known to man.

Today, smallpox is a potential weapon of mass destruction that could be wielded against the U.S. by enemies like Iraq and al Qaeda.

With that in mind, President Bush is expected to announce on Friday a plan which will gradually make the smallpox vaccine available to all Americans who want it.

That's according to administration sources who say the shots will be mandatory for about 500,000 military personnel and recommended for another half-million who work in hospital emergency rooms and on special smallpox response teams.

The general public will be offered the vaccine on a voluntary basis as soon as large stockpiles are licensed, probably early in 2004, though the government will not encourage people to get them.

60 Minutes II Correspondent Dan Rather reports that in evaluating the potential danger of smallpox, the Bush administration has faced a deadly dilemma: Do not vaccinate the population against small pox and leave millions of people vulnerable to one of the worst scourges known to man. Or treat people with a vaccine that is extremely effective at blocking the disease but can cause dangerous, sometimes fatal, reactions.

The United States stopped giving mandatory smallpox vaccinations 30 years ago. Soon after that, doctors eradicated the disease from the planet. But now, the government has decided to bring back the vaccine because of fear that terrorists, or Iraq, could use the virus as a weapon.

But smallpox was, or is, a terrible, virulent disease. It kills one out of every three of its victims. There is no cure.

The vaccine effectively immunizes against smallpox. But that protection has a price. Some people die from it; and others have serious reactions, some permanent. Scientists say it's the most dangerous vaccine known to man.

It could protect Americans from the unthinkable destruction of a smallpox attack. But the vaccine has a dark side.

"We know if we immunize a million people, that there will be 15 people that will suffer severe, permanent adverse outcomes and one person who may die from the vaccine," says Dr. Paul Offit, one of the country's top infectious disease specialists, and he knows all about vaccines that prevent those diseases. In his lab at Children's Hospital of Philadelphia, he studies and creates new vaccines. There's nothing new about the smallpox vaccine.

The vaccine was created in 1796. The vaccine used today is essentially the same, Offit says. "We tend to think of vaccines as being very safe and every effective, which they are. But all the vaccines that we use today are the result of modern technology. That's not true of the smallpox vaccine. It has a side effect profile that we, we would not accept for vaccines today," he says.

The smallpox vaccine is made from a weak biological cousin of the smallpox virus. When you get vaccinated with the weaker virus, you become immune to the smallpox virus.

But once in a while, the vaccine does more harm than good. If you scratch where the smallpox is at the surface, and you put it to the eye, you can transfer the smallpox to your eye. That occurs in about 500 people for every million that get the vaccine. If you get "progressive vaccinia," your immune system is compromised. The virus just continues to grow and grow, and is often the cause of death.

No one is certain how many people will be hurt by the vaccine. A 1969 study found that, out of every one million people vaccinated, 74 will suffer serious complications, and at least one will die.

These side effects were never a secret, but they were rarely discussed, when the law required every child to get a smallpox vaccination before starting school.

Previous generations, Offit says, faced the disease regularly, and knew its power. "My parents, they saw diptheria. They saw polio. They saw the plagues that devastated children in this country. And so, the notion of getting vaccinated was, you know, nothing compared to what they were seeing during their normal day," he says.

If you did get the smallpox vaccine as a child, it may help you today. Some experts think you may still have some of your old immunity against the disease. But that's only a theory. There are no reliable studies on this, and other scientists disagree.

So why is the government recommending a dangerous vaccine that could kill people?

Because the disease it prevents is worse.

"It is the worst human disease. It probably killed more people in history than any other infectious agent, including the Black Death of the Middle Ages," says Richard Preston, who writes about deadly diseases like Ebola and anthrax. Nothing scares him like smallpox. He's just written a book called "The Demon in the Freezer," about how the smallpox virus has been turned into a weapon of mass-destruction.

"Smallpox as a weapon is the biological equivalent of the nuclear bomb. It is simply the most dangerous biological weapon in the world," he says. That's because it spreads on its own, unless you stop it.

Says Preston: "There is a heated debate going on in the scientific community right now about such things as, for each victim of smallpox, how many people are going to catch it from each victim? That's known as the multiplier of a virus. If the multiplier of smallpox is 10 - that is to say, if each person infected with smallpox on average gives it to another 10 people - then a smallpox outbreak would be explosive in our society."

The multiplier was much higher than 10 in a small town in Germany in 1970. One man was hospitalized with the disease, and kept in an isolation ward. But 19 hospital staff and patients, who never saw the man, got smallpox. Still, doctors were able to control that outbreak with what is known as "ring vaccination."

"We know how to put a ring around the infection to contain it. You identify a person who's infected, you quarantine them, you isolate their contacts, and then the contacts of those contacts. And that eliminated smallpox from the face of the earth," says Offit.

Offit thinks it's a mistake to vaccinate lots of people now, before there's any kind of outbreak. He thinks there's a safer approach: "Here's another way to do it. We can make the vaccine. Make sure we understand who's going to get it, who's going to be giving it. Then wait, wait for there to be one case of documented smallpox somewhere on the face of this earth and then we can move into vaccinating people, large numbers of people."

Many public health specialists who worry about vaccine side-effects say that only a few people should be vaccinated until there is an outbreak. But others are more fearful of a smallpox attack.

"You see very good physicians arguing very hard that smallpox is really not a problem, and that it can be easily handled. Those doctors are maintaining an excellent bedside manner with the American people, but they don't fully inspire confidence in me," says Preston.

Israel wants to be prepared for a smallpox attack. In August, they immunized nearly 15,000 health-care workers. More vaccinations are planned. So far, there have been four bad reactions, two very serious. And some health care workers are unwilling to be vaccinated.

In the U.S., doctors are conducting a scientific study to find out just how people react to smallpox vaccines. Dr. Gregory Poland's staff at the Mayo Clinic started inoculating volunteers just a few weeks ago.

One of Dr. Poland's major concerns is making sure people don't spread the vaccine virus to other parts of their bodies, or to other people who haven't been vaccinated.

"What if a child touches it? A pregnant woman? Somebody with HIV infections? They could potentially die as a result," he says. But Dr. Poland admits that the consequences of a real smallpox attack are frightening.

"If there was a large population outbreak of smallpox because our tools are so limited, and we really don't have anything to treat them with, it would be comfort care because that's all there is to do, and they either recover or they die," he says.

The New York City Health department has been working for months to figure out exactly who should get inoculated, and when. Dr. Thomas Frieden, the health commissioner, estimates his department will have to vaccinate 15,000 New Yorkers very soon, and perhaps 300,000 more in coming months.

"The vaccine is not for the general public. It's only for those people who would be - in the case of a smallpox outbreak - would be responding to, and caring for, the initial cases," he says.

The first to be vaccinated, Frieden says, will be a team of health care workers at each hospital in New York. But some people should not be vaccinated.

"Some of the people who shouldn't get the smallpox vaccine are those who have a weakened immune system, including those who are HIV positive, those who have gotten cancer treatments, and those who have a transplant," he says.

Also included on that list are people with the skin-condition eczema, pregnant women and infants. Frieden's department has real-life experience with large-scale vaccination programs. More than 50 years ago, a tourist brought smallpox virus to the city, and health officials reacted quickly.

More than six million people were vaccinated in three weeks in New York City in 1947. There were 12 cases of smallpox. "There were two deaths associated with smallpox, and there were, I'm sorry to say, three deaths associated with the vaccine," he says.

"(But) if they hadn't vaccinated, they might've had thousands upon thousands of deaths from smallpox," he adds.

It comes down to odds. Scientists know what the risks are for the vaccine, but no one knows - or is willing to say - what the chances are of a smallpox bio-terror attack. That leaves us with a dangerous gamble.

"I feel that someday we are quite likely to see smallpox again. Because it's held in human hands. And human hands are weak. And the human heart has dark corners to it," says Preston.

According to Offit, the country will have enough vaccine to immunize everybody by the middle of next year.