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It’s not often that we think of an organism becoming extinct as being a good thing. However, this is the case for smallpox.

Today marks the 33rd anniversary of the World Health Organisation passing a resolution declaring that smallpox had been eradicated. Smallpox had been around for thousands of years – scars found on the mummy of Egyptian pharaoh Rameses V suggest he may have had the disease.

Smallpox was caused by the variola virus. It was highly contagious, and symptoms include fever, headache and back pain. It was also known for causing a rash of blisters that could cover the whole body. The most common strain of the virus, variola major, had a high mortality rate, killing up to 40% of people who contracted the disease. Even those who survived were sometimes left blind and disfigured by the disease.

The eradication of smallpox was made possible due to vaccination. Over 200 years it was observed that people who caught cowpox, a similar but much less deadly disease, didn’t catch smallpox. By exposing people to the cowpox virus, it became possible to prevent smallpox. This was the first example of vaccination.

Vaccination is still an important part of medicine today, and is used to prevent a range of diseases, including measles, whooping cough and chickenpox. Smallpox still remains the only infectious disease that affects human to have been eradicated. Polio is another once-common disease that has been greatly reduced, with vaccination playing an important role. Could polio be next?