A handful of Europeans conquered the Americas. Their most devastating weapon was one they did not even know existed.

The Aztec Empire of Mexico was one of the most sophisticated on earth. Then, in the spring of 1519, Hernán Cortés and his small band of Spanish 'conquistadors' reached the country. Everything was about to change. They had guns, they had swords, but they had another, even more powerful weapon – infectious diseases to which the Aztecs had never been exposed.

Between 1520 and 1521, between 10 per cent and 50 per cent of the population of the capital of the Aztec empire was wiped out by an infection of smallpox. Epidemics of smallpox and typhus subsequently spread through the country and within 50 years of the Spanish invasion, the population of Mexico probably fell from around 30 million to just 2 million. Smallpox had an even more devastating impact on the Incas of Peru, drastically depleting their numbers several years before Francisco Pizarro and his conquistadors reached them.

The story was similar in North America, where diseases from Europe wrought havoc among the native Americans. Some reckon that Old World diseases like smallpox may have been responsible for killing up to 95 per cent of the New World population.