For centuries, smallpox killed millions of people around the world. But thanks to global immunization programs, the deadly infectious disease was wiped out in the late 1970s.
Today, scientists keep only a small amount of the virus alive under tightly controlled conditions in the U.S. and Russia for medical research.
Routine smallpox vaccinations stopped in the U.S. and in many other countries in 1972, and in all other World Health Organization member countries by 1986. Many adults living today likely got the vaccine as children.
What Causes Smallpox?
The variola virus causes it. There are two forms of the virus. The more dangerous form, variola major, led to smallpox disease that killed about 30% of people who were infected. Variola minor caused a less deadly type that killed about 1% of those who got it.
How Smallpox Is Spread
The disease is highly contagious. You get it mainly by breathing in the virus during close, face-to-face contact with an infected person. It usually spreads through drops of saliva when the person coughs, sneezes, or speaks.
Smallpox also can spread when someone handles the clothes or sheets of an infected person or comes into contact with their body fluids. Very rarely, smallpox has spread among people in small, enclosed spaces, probably through air in the ventilation system. Animals and insects don’t spread the disease.
Once a person is infected with the virus, 7 to 17 days can pass before they have any symptoms. During this time, the person isn’t contagious and can’t spread the virus to others.
An infected person is most contagious once they start having symptoms. He can spread smallpox to others until he is completely symptom-free.
Smallpox gets its name from its most common sign of the disease: small blisters that pop up on the face, arms, and body, and fill up with pus.
Other symptoms include:
- Flu-like fatigue, headache, body aches, and sometimes vomiting
- High fever
- Mouth sores and blisters that spread the virus into the throat
- A skin rash that gets worse in a typical pattern:
- The rash starts with flat red sores that become raised bumps a few days later.
- The bumps turn into fluid-filled blisters.
- The blisters fill with pus.
- They crust over, usually in the second week of smallpox.
- Scabs form over the blisters and then fall off, usually in the third week of the disease. They can cause permanent scars.
- Blindness can happen when blisters form near the eyes.
There’s no known treatment for smallpox. Getting the vaccine within 3 to 4 days of contact with the virus may make the disease less severe or maybe help prevent it.
Beyond that, medical care aims to ease symptoms like fever and body aches, and control any other illnesses that a person can get when their immune system is weak. Antibiotics can help if someone gets a bacterial infection while they have smallpox.
Scientists are still looking for antiviral medicines that could treat the disease. The drug cidofovir has worked well in early studies.
Prevention: The Smallpox Vaccine
Scientists use the cousin virus to variola -- the vaccinia virus – to make the smallpox vaccine, because it poses fewer health risks. The vaccine prompts the body's immune system to make the tools, called antibodies, it needs to protect against the variola virus and help prevent smallpox disease.
No one knows for sure how long the smallpox vaccine protects people from the disease. Some experts believe it lasts for up to 5 years and wears off over time. Since it may not give lifelong protection, anyone vaccinated years ago as a child could be at risk of future infection by the variola virus. The only people known to be immune for life are those who have had smallpox and survived.
The World Health Organization and its member countries keep an emergency stockpile of the smallpox vaccine. It’s rarely used today, except for those few people who are around the variola virus, such as laboratory researchers working with variola and viruses like it.
Risks of the Vaccine
Some of its side effects can be dangerous, especially for people with weak immune systems. They can range from skin reactions to a serious nervous system condition called encephalitis, which can lead to convulsions, coma, and death. But these side effects are very rare. Based on historical data, for every 1 million people vaccinated for smallpox, one to two people died from a bad reaction.
Some people would have a higher risk of a reaction to the vaccine, like:
- Women who are pregnant or breastfeeding
- People with skin disorders such as eczema
- People with a weak immune system due to a medical condition like leukemia or HIV
- People on medical treatments, such as for cancer, that make the immune system weak
Smallpox as a Public Health Threat
It’s hard to know how major a threat a smallpox outbreak would be today. There are a few reasons that scientists can’t be sure:
- The number of people around the world with weakened immune systems is higher today than when smallpox existed.
- Countries used vaccines of different strengths during the global effort to end smallpox.
- There’s no way to know for sure how long these different vaccinations give immunity to the virus.
If an outbreak of smallpox were to happen, public health measures would likely include these steps: find and vaccinate infected people, vaccinate health care workers and others at risk of infection, isolate smallpox patients to keep them from spreading the disease, and give vaccinations for the public as needed to contain the outbreak.