Smallpox is the only human disease to be eradicated - here's how the world achieved it

Until today smallpox is the only human disease that has been successfully eradicated.¹ The eradication of smallpox is therefore a major success story for global health for several reasons: it was a disease that was endemic – and caused high mortality rates – across all continents; but was also crucial to advances in the field of immunology as the smallpox vaccine was the first successful vaccine to ever be invented.

Smallpox is an infectious disease caused by the variola virus which infects exclusively humans. No treatment existed so once you caught smallpox nothing could be done but let the infection run its course. Smallpox was responsible for millions of deaths in the past. The chart here illustrates this by visualizing the deaths due to smallpox as a share of all deaths in London from 1629 to 1902. In peak years, almost every fifth death was caused by smallpox! Death rates were similarly high in other European countries.

In 1796, British surgeon Edward Jenner invented the smallpox vaccine, the world's first vaccine against any disease. The dramatic decrease in smallpox deaths at the turn of the 19th century can be credited to his invention. Our entry's section on how eradication was achieved, we discuss how Jenner discovered the virus that protected people against smallpox infections and how he initially struggled to spread the word about it.

Thanks to the widespread roll out of Jenner's vaccine across Europe and North America, smallpox was almost entirely eliminated in these regions by the first half of the 20th century. When the WHO launched its 'Intensified Smallpox Eradication Program' in 1966, most cases were therefore restricted to South America, Africa and Asia – especially India –, where the vaccine's rollout had been slower and patchier.

The map here shows the number of smallpox cases by country – note the large differences in color brackets that were necessary to accommodate China and India's large case numbers. Large-scale vaccination campaigns however meant that the remaining countries saw rapid declines in the late 1960s and early 1970s. You can use the 'play' button at the bottom of the map to see how case numbers evolved over time and by clicking on individual countries, you can see their reported smallpox cases in a line chart.

The more severe type of smallpox, caused by the variola major virus strand, was eradicated in 1975 in Bangladesh, shortly after India had been declared smallpox-free that same year. The variola minor virus, which caused a weaker form of smallpox infections, was still in circulation on the African continent, though. The last smallpox infection worldwide was documented in October 1977 in Somalia.

The map below shows the year in which smallpox ceased to be endemic for each country, with each decade corresponding to a different color.

The chart here illustrates how deadly the disease once was. Just the reported number of smallpox cases between 1920 and 1978 already amounted to 11.6 million cases; and that number was certainly smaller than the actual number of cases, although we don't know by how much. We discuss estimates of this discrepancy in the data quality section of our entry.

In the 1950s and 60s the number of reported cases started to decline and by the end of the 1970s there were no cases reported anymore. The WHO search teams continued to search for more smallpox cases but, with the exception of two tragic cases in Birmingham in the United Kingdom due to a laboratory accident in 1978,² found none. Therefore, in 1980 the World Health Organization declared smallpox the first – and so far only human – disease to be eradicated globally.

Imagine what this means, a disease that was once common across the world and caused millions of deaths and disfigured the faces of even more simply does not exist anymore.

For more detailed information on the characteristics of smallpox, its global history, and the vaccine that ended the disease, visit our entry on Smallpox that we published simultaneously with this blog post.