Medical Conditions - Smallpox

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The Facts

Smallpox was an infection that was caused by the virus called variola virus. For thousands of years, smallpox created severe illness and caused the death of hundreds of thousands of people. When it was introduced into the Americas from Europe in the 1500s, it killed many of the native populations. As late as the 1800s it was still causing the deaths of thousands when introduced into susceptible populations, such as in Hawaii, by European explorers. Fortunately, this virus was eliminated as a natural cause of disease in 1977 through effective use of vaccination programs. It is the only disease ever to be deliberately removed from the human population.

It is thought, however, that this virus could be reintroduced as an agent of biological warfare. This is because the virus is very contagious (can be spread from person to person) and can cause serious illness, even death, if an individual is not vaccinated within four days of being exposed to the virus. Since antibiotics only work against bacteria, not viruses, they are ineffective against the smallpox virus. There is no known cure.

Prior to 1972, vaccination against smallpox was standard procedure. Since that time, general vaccination against smallpox has not been recommended and therefore has been unavailable to the general population. It is thought that protection gained from the vaccination against smallpox lasts a maximum of 10 years. It is believed that few people in North America currently have protection against this disease. In light of current terrorist activity, the American and Canadian governments have acquired their own emergency supply of smallpox vaccine. These vaccinations would be used to vaccinate any people exposed to the smallpox virus should this become necessary.


The smallpox virus is passed from one person to another through inhalation of air droplets or aerosols. Therefore, it is important to isolate a person diagnosed with smallpox and not have face-to-face contact. The disease is spread most easily during the first week of infection, but the risk of passing the virus to another person lasts until all scabs have fallen off (see "Symptoms and Complications"). Clothing or bedding can also spread the virus.

As a weapon of biological warfare, it is most likely that the smallpox virus would be spread through the ventilation systems of buildings. The viruses would only stay alive for one or two days within the building, but by that time many people may have become infected. Since it takes about 2 weeks for the symptoms of smallpox to develop, it is unlikely that the source of the infection would be discovered in time to do anything about it. Smallpox can be passed between people in any climate, but it spreads most easily in cool, dry winter months. It is believed (and hoped) that the only existing smallpox virus in the world is located at two WHO (World Health Organization)-designated laboratories: the Centers for Disease Control and Prevention in the US and Russia.

Symptoms and Complications

On average, the time between coming in contact with the smallpox virus and developing symptoms (the incubation period) is 12 days. The incubation period ranges from 7 to 17 days.

The first symptoms of smallpox are high fever, fatigue, headaches, and backaches. A rash that appears mostly on the face, arms, and legs starts about 2 to 3 days later. The rash starts out as a flat, spotty (papular) rash, but soon develops small vesicles (clear fluid-filled lesions) and then later the lesions become pustular (filled with cloudy white pus). The pustules (pus-filled lesions) are usually deeply embedded in the skin and are round and hard. As the pustules continue to get bigger, the person with smallpox is usually in a lot of pain, and the high fever continues. Crusts begin to form on about the eighth or ninth day of rash. Scabs form and separate, leaving deep, pitted scars.

Death from smallpox typically occurs in about 30% of unvaccinated people developing the common variola major form of the disease. In those who have been vaccinated, the death rate is about 3%. One variant of the disease is called variola minor, and the death rate from it is less than 1%. These estimates come from the rates of death before 1972 (the last naturally occurring case of the disease). Many of the deaths occurred from bacterial infections getting into sores; good wound care is thus extremely important. Death from smallpox usually occurs during the second week of symptoms.

Making the Diagnosis

Smallpox is initially suspected through appearance of the symptoms described above. The early symptoms (first 2 to 3 days) of smallpox could be confused with chickenpox and other similar diseases. The differences are that the chickenpox rash is denser over the trunk, while the smallpox rash tends to occur to a higher degree on the face, arms, and legs.

The symptoms of smallpox are much more severe (e.g., high fever, muscle aches) than those of chickenpox, and the entire rash associated with smallpox evolves at the same rate – from all the lesions being papules to all becoming vesicles, to all becoming pustules at the same time. In chickenpox, the rash occurs in various stages – in some areas there are papules, in others vesicles, and in others pustules – but all 3 types can be seen to be present at the same time.

To confirm the diagnosis of smallpox, a health official wearing gloves and a mask collects a piece of scab or the fluid inside the vesicles or pustules. The diagnosis can be quickly confirmed in the laboratory through use of an electron microscope and confirmed by culturing the virus from the scab or fluid.

Treatment and Prevention

There is no known specific treatment for smallpox, although research is being carried out in an attempt to create a medication capable of killing the virus. Control of fever, prevention of dehydration, good wound care, and antibiotics for any secondary infection caused by bacteria are important measures for the smallpox patient.

If a case of smallpox were diagnosed, that person would be isolated from other people immediately, and all people that had been in contact with that person would be isolated, vaccinated, and watched closely for signs of the disease. Vaccine that is given within 4 days of exposure to the virus may prevent or greatly reduce the symptoms of the disease.

Currently, smallpox vaccine is approved for use only with those people who are at special risk for the disease. Laboratory workers working with the virus are also vaccinated. If an epidemic were to occur, widespread vaccination could take place, with priorities as recommended by the government.

The vaccination against smallpox contains the cowpox (vaccinia) virus, not the smallpox virus. The vaccinia virus, which is related to the smallpox virus, allows the production of antibodies against the smallpox virus by our body’s defense (immune) system. The vaccine is very safe, and was routinely given to the general population before 1972. Certain people are at higher risk for adverse reactions to the vaccine – most reactions are mild, but some are severe. They include people with eczema or other skin conditions, pregnant women, and persons with circumstances that result in weakened immune systems, for instance those with cancers like leukemia and lymphoma and people who have had solid organ transplants.

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