Smallpox is a highly infectious and extremely deadly disease caused by the Variola virus. It kills a third of those it infects.
The vaccine is so effective, it medical personnel to eradicate the disease. Once it was gone, routine vaccinations stopped. Since the protective effect wears off over time, everyone, including someone who was vaccinated in childhood, is now vulnerable.
Fears about smallpox have galvanized a huge effort to stockpile smallpox vaccine and to prepare emergency plans, should an outbreak be detected. The good news is that it looks as if the existing inventory of vaccine can be diluted to provide effective doses for millions of people, if necessary. Also, the government has ordered more vaccine to make up any shortages.
The question is who should get the vaccine and when.
The CDC is recommending that half a million hospital workers be vaccinated as a precaution. The final decision has not been made by the president yet. He has to decide whether to also include up to ten million more emergency responders, such as police and paramedics, or to vaccinate the entire population.
There are already plans to vaccinate about half a million military personnel.
There is a reluctance to vaccinate everybody because the vaccine is made from a potentially dangerous live virus called Vaccinia. If everyone in the U.S. were routinely vaccinated, it's estimated that hundreds would die each year and thousands would become severely ill.
Even the more common side effects include fever, headache, nausea, muscle aches, lesions, pain and swelling.
The safety issue is once again at the forefront of the debate, with a new proposal to test the safety of the vaccine in toddlers and preschoolers. There are many who believe that the risk of a smallpox attack is not high enough to warrant the use of a potentially dangerous vaccine in otherwise healthy people, especially children.
People who get the vaccine are also contagious for a time and could spread it unknowingly to people who haven't been vaccinated. Vaccination of select people has to be done carefully to avoid infecting others.
Routine smallpox vaccinations ended in the U.S. in 1972, and most experts believe that those last vaccinated 30 years ago have little immunity remaining.