Smallpox

  • Medical Author:
    Mary D. Nettleman, MD, MS, MACP

    Mary D. Nettleman, MD, MS, MACP is the Chair of the Department of Medicine at Michigan State University. She is a graduate of Vanderbilt Medical School, and completed her residency in Internal Medicine and a fellowship in Infectious Diseases at Indiana University.

  • Medical Editor: Charles Patrick Davis, MD, PhD
    Charles Patrick Davis, MD, PhD

    Charles Patrick Davis, MD, PhD

    Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

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What causes smallpox?

Smallpox is caused by a poxvirus called variola (Poxviridiae family of viruses). Variola is a relatively large virus that contains double-stranded DNA. The virus can be found in large numbers in many organs (skin, kidneys, spleen, liver, and other organs). Death occurs because of overwhelming toxemia, thought to be due to immune complexes trying to react to the large number of viral particles. Variola infection only occurs in humans, which was helpful in eradicating the disease. There are two strains called respectively variola major and variola minor (also known as alastrim). As implied by the names, variola major is more likely to cause serious disease and death than variola minor.

How is smallpox transmitted?

Most transmission of smallpox is directly from person to person. Large, infectious droplets of saliva are expelled during coughing or sneezing and then inadvertently inhaled by another person. This usually requires close face-to-face contact and is similar to the way that mumps, measles, and influenza are spread. On average, a single individual would infect approximately 60% of their household contacts. Infected objects, such as used silverware or heavily contaminated bedding, may carry sufficient numbers of organisms to infect another person if improperly handled, although this route of transmission is much less common.

What are smallpox symptoms and signs?

Symptoms develop seven to 17 days after exposure. Fever is the most common initial symptom and can be quite high. This is accompanied by body aches. Often, the patient is too unwell to get out of bed. Within 24 to 48 hours, a rash begins to appear everywhere on the body but especially on the legs, arms, mouth, and face. The eyes may also be affected, leading to potential blindness among survivors. Symptoms in children are similar to adults. The rash also appears on the palms and soles and goes through stages as the disease progresses. At the beginning, the rash consists of red dots that become raised. The lesions (see Figure 1) rapidly fill with fluid resembling a blister or cold sore and are known as "vesicles." The fluid in the vesicles may turn yellow, resembling pus. Rarely, the rash may start to fill with blood (hemorrhagic smallpox), which is a poor prognostic sign. After one to two weeks, the lesions scab over and eventually fall off, leaving deep scars. One of the defining features of smallpox is that all the lesions on the body are always at the same stage of development. This is in contrast to chickenpox where new lesions form while old ones are healing.

Picture of smallpox in a young child.
Figure 1: Picture of smallpox in a young child. SOURCE: Dr. Michael Schwartz/CDC

Approximately one-third of people with smallpox died from the infection. People who had an extensive rash were at higher risk to die. People who had only a few lesions or a milder rash had a lower risk of death. Infections caused by the variola minor strain were less severe and death occurred in only approximately 1% of cases.

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