Smallpox – the only infectious disease we've ever eradicated

It was feared for millennia and among the most deadly diseases facing humankind but today smallpox is no more. It is the only infectious disease that’s been successfully eradicated.

The disease, which only affects humans, is thought to have killed more than 300 million people and was fatal to around 30% of those who became infected. Survivors were often left disfigured by pockmarks and scars. Some went blind.

Smallpox is likely to have proliferated in densely populated towns of early civilizations before being carried along trade routes and on ships until it had spread worldwide. It continued to plague humanity for more than 3,000 years,

Then in the late 18th century a vaccine was discovered. And thanks to a series of programmes designed to eradicate the disease – which involved identifying all cases and their contacts and ensuring that they were all vaccinated – it was eliminated in the second half of the 20th century.

What is smallpox?

The disease is caused by the variola virus. Its most notable outward signs are a rash that forms on the face, arms and legs of an infected person although it starts – like many viral infections – with an elevated temperature and tiredness.

The rash develops into clusters of fluid-filled pustules, which can look superficially similar to the chickenpox rash, although the two diseases are caused by different viruses.

Early symptoms include high fever and fatigue. The spots on the skin become filled with clear fluid and later, pus, and then form a crust, which eventually dries up and falls off.

The virus spreads in droplets, much like coronaviruses, and can be expelled into the air via coughing and sneezing. But it can also be passed on via the fluid from smallpox blisters. Typically, someone would need to have been in close proximity to an infected person before catching it themselves.

Vaccination

There are no effective treatments or cures for smallpox, but vaccination can be used effectively to prevent infection from developing.

The earliest attempts at immunization involved a process known as variolation. This involved taking a small sample of the fluid from an infected person’s rash and – using a small, sharp bladed instrument called a lancet – placing that fluid under the skin of another person.

The procedure wasn’t without risks. The recipient would usually make a full recovery and develop immunity to smallpox. But they may also have been shedding the virus and spread the disease to people who subsequently became seriously ill and died.

Two people in particular are credited with developing what we now think of as the smallpox vaccine.

The first was farmer Benjamin Jesty, who spotted a link between smallpox and cowpox. More than two decades later there was Edward Jenner, a doctor from England who is usually credited with the discovery and whose rigorous, scientific approach helped the idea of vaccination take off more widely.

He observed that agricultural workers who had contracted cowpox seemed to be immune to smallpox. In 1796, Jenner set out to prove there was a connection between these two illnesses. He found a cowpox sufferer, extracted fluid from one of her blisters and – using the variolation technique – infected an eight-year old boy.

Jenner’s subsequent attempts to infect the boy with smallpox – by exposing him to the disease – all failed, demonstrating that the small dose of cowpox led to immunity from smallpox.

Eradication

The smallpox vaccine is not routinely administered to anyone who isn’t at risk.

Mild reactions to the vaccine can include fever, fatigue and tiredness. But it carries a small risk of serious side-effects, including a condition called progressive vaccinia, where the vaccine triggers an uncontrolled replication of the virus at the vaccination site. Surrounding skin and flesh tissues then slowly starts to die. Untreated, it can lead to organ damage or failure, and ultimately death.

For this reason, contact tracing and selective vaccination have been key in the eradication of smallpox. In the US, routine strategies for the event someone is diagnosed with smallpox centre around isolating the infected person and tracking their movements over the previous three weeks.

Healthcare workers dealing with the infection will be vaccinated within 72 hours of contact, as will all members of the infected person’s household. All affected people will be monitored for at least 14 days for signs of the disease.

By tracking down common points of contact, it is possible to isolate pockets of infection and eradicate them without the need to roll out generalized vaccination programmes to the entire population.

The last known natural case of smallpox was in Somalia in 1977. The following year, an accident in a research laboratory led to the death of one person from the disease, and in 1979 smallpox was officially declared eradicated.