Condition Basics

What is smallpox?

Smallpox is a contagious infection caused by the variola virus. Smallpox can be deadly, so if an outbreak happens, it is vital to stay away from infected people. Get vaccinated if you have been around someone who has smallpox. And if you have symptoms, seek medical care.

The telltale signs of smallpox are severe illness with a high fever, then a body rash. Symptoms appear about 12 days after the person is infected.

Before there was a vaccine, smallpox used to cause death all around the world. Thanks to widespread use of the vaccine, the last natural case of smallpox occurred in 1977. And in 1980 the World Health Organization (WHO) declared that the spread of smallpox was stopped and that the disease had been wiped out.

Because there is a slight risk of serious reactions and even death from the smallpox vaccine, routine smallpox immunization ended in the United States in 1972.

Smallpox virus is known to exist in labs at the Centers for Disease Control and Prevention (CDC) in the U.S. and at the State Research Center of Virology and Biotechnology in Russia. But it may also be in other labs. Some people worry that terrorists could release the virus and spread smallpox to many people.

How is it spread?

Smallpox is contagious. It can be passed from one person to another through coughing, sneezing, or breathing, or by contact with the scabs or the fluid from blisters. It can even spread from an infected person's personal items and bedding. Smallpox is easiest to spread during the first week of the rash. As scabs form, the person is less contagious. But a person can spread the virus from the time the rash first appears until all scabs have fallen off.

If a terrorist were to release a small amount of the virus into the air, it is possible that it could spread among a large number of people. The virus may be able to survive and infect people for up to a day.

People who get this disease must stay away from others to help prevent it from spreading. If there has been a smallpox outbreak and you think you might have been exposed, call your doctor or local health department. Do not go directly to a health facility, because you could pass the disease to other people.

What are the symptoms?

The first symptoms of smallpox include a high fever, fatigue, a headache, and a backache. After 2 to 3 days of illness, a flat, red rash appears. It usually starts on the face and upper arms, and then it spreads all over your body. Over the next 2 to 3 weeks, the flat, red spots become firm and dome-shaped and fill with pus. Then they scab over. Scabs fall off 3 to 4 weeks after the rash first appears, and they leave pitted scars.

The incubation period for smallpox is about 12 days. But symptoms can develop as soon as 7 days or as long as 19 days after exposure.

You may mistake a severe chickenpox rash for a smallpox rash at first. But different viruses cause these illnesses and there are many differences between chickenpox and smallpox rashes.

How is it diagnosed?

If a doctor suspected a case of smallpox, blood and skin tests would be needed to confirm the diagnosis. A confirmed case of smallpox would be considered a worldwide health emergency. In the U.S., state and federal health officials would quickly take action. They would keep anyone who might have been exposed away from others.

If a smallpox outbreak had been confirmed, a doctor in the outbreak area could diagnose smallpox without a lab test. The doctor would look at the rash and ask about symptoms and possible exposure to the disease.

How is smallpox treated?

There is no known cure for smallpox. Treatment includes drinking plenty of fluids and taking medicines to control pain and fever. To prevent the spread of the virus, an infected person must be kept away from other people until he or she is no longer contagious.

How can you prevent it?

People who have survived smallpox cannot get it again.

Also, there is a smallpox vaccination. It has vaccinia virus in it, which is like the smallpox virus but safer. If you get the shot before you've been exposed to smallpox, it will likely protect you for at least 3 to 5 years. And having a second shot later can protect you for an even longer period of time.

The shot works even if you don't get it in advance. Most people who get the smallpox shot within 3 days after they've been exposed to the virus will have no symptoms or will have symptoms that aren't as severe. Getting a shot 4 to 7 days after exposure may also help.^{footnote 1}

People who have very close contact with a person who has gotten a smallpox vaccine can get an infection from the virus used in the vaccine. The infection usually causes a minor skin rash and is not smallpox. So the site where the smallpox vaccine was given should be covered until the scab falls off.

In the past, when a smallpox infection was diagnosed, infected people were kept away from others to prevent the spread of infection. Everyone who might have been exposed to the virus was then vaccinated. This practice, called ring vaccination, played a key role in wiping out smallpox. Many experts think it would be better to carry out ring vaccination before mass vaccination if there were a case today.

Because there are risks of a serious reaction from the vaccine, routine smallpox immunization doesn't occur. All children and most adults in the U.S. today have a chance of getting infected if they are exposed to the smallpox virus.

Since the September 2001 terrorist attacks on the U.S., more vaccine has been made. The U.S. government has enough smallpox vaccine for all Americans in case of an outbreak.^{footnote 1}

The smallpox vaccine is recommended for laboratory workers who handle the vaccinia virus, for members of smallpox response teams, and for certain people in the military. For accurate, up-to-date information, visit the Centers for Disease Control and Prevention (CDC) website at www.bt.cdc.gov/agent/smallpox.