When populations increase and come into contact with other population, they often encounter diseases for which they have no immunity – and the results can be devastating. There are many examples.

  • In 430 BC when the Athenians encountered the Persians in the Peloponnesian Wars, they also encountered typhoid fever which killed a quarter of the population of Athens in the next four years.
  • Around 165 AD, Roman soldiers returning from the Middle East brought smallpox to Italy – and five million people died from the disease.
  • In the 530s, Justian’s Byzantine armies conquered North Africa. In 541, bubonic plague, probably brought by rats in wheat shipments from Egypt, broke out in Constantinople, killing possibly 80% of the city’s inhabitants. It spread throughout the known world, eventually reducing Europe’s population by 40%.
  • Bubonic plague was again brought to Europe in 1348, probably by Italian traders returning from the Crimea. Between 20 and 30 million Europeans died.
  • Diseases, such as smallpox and measles, brought by Europeans to South America in the 16th century, wiped out up to 90% of the native poulation.
  • British traders and soldiers brought cholera from India to London in 1832 and spread it to the Americas by 1834.
  • Spanish flu, spread by American troops during the First World War, killed between 50 and 100 million people. In Fiji, 14% of the population died in just two weeks.

We now have much larger populations and much more contact between them. HIV/Aids has killed more than 25 million people and over 30 million now have the disease. Several other diseases have the potential to become pandemics – SARS, bird flu, Lassa fever, Rift Valley faver, Marburg virus, Ebola virus and Bolivian hemorrhagic fever are all highly contagious, deadly diseases.

At the same time antibiotic-resistant “superbugs” could allow the re-emergence of diseases, like tuberculosis, which are currently controlled – approximately 50 million people worldwide are already infected with forms of TB which are multi-drug resistant.

Furthermore, there are fears that global warming could mean that diseases like malaria, which are now largely confined to tropical regions, could spread more widely. Malaria currently causes between one and three million deaths a year.

Worldwide distribution of malaria (Public domain image)
Worldwide distribution of malaria
(Public domain image)