European germs

In 2004, archaeologists in Peru found a gun-shot wound to the head killed at least one Incan during the Spaniard Pizarro's invasion of the 1530s. This is apparently the first (of many) gunshot wounds recorded in the Americas.

According to J. Diamond's Guns Germs and Steel, there were many things going around more dangerous than musket shot. The Incan empire was still reeling from a bloody internal struggle arising after an epidemic of smallpox created a power vacuum. Smallpox has wiped out large populations across the globe as Europeans arrived on foreign shores and met those with no immunity. It was a major cause of death to Australian aborigines during the 1800s. Was the disease's introduction here deliberate? I can't seem to find an authoritative source. Opinions are divided.

Back to the Incas: Atahuallpa had taken control of the Incan empire but did not rule for long. With around 200 steel-armed and armoured men, some cavalry and a few (lousy but frightening) guns, Pizarro took the unprepared ruler captive and started slaughtering countless numbers of his 80,000 amassed troops. According to Spanish sources there were no Spanish casualties. Despite paying a huge ransom Atahuallpa was slaughtered. Without its figurehead, without a written history of thousands of years of conflict to fall back on for advice, with no guns or steel, even the best laid Incan rebellions were dismissed easily by Spanish troops.

Would a release of Smallpox be as deadly to us as it was to the Inca people and Australian aborigines? You bet. See Transmission potential of smallpox in contemporary populations in Nature. Plague, a fascinating and terrifying thing.