McNeill's law

In human geography, McNeill's law is the process outlined in William H. McNeill's book Plagues and Peoples. The process described concerns the role of microbial disease in the conquering of people-groups.

According to McNeill's Law, the microbiological aspect of conquest and invasion has been the deciding principle or one of the deciding principles in both the expansion of certain empires (as during the emigration to the Americas) and the containment in others (as during the crusades).

Instances in History
The first people-group fully wiped out due to European expansion (with the possible exception of the Arawaks) was the Guanches of the Canary Islands. Despite an inbred ferocity, superior knowledge of the land and even a possible tactical superiority, they were eventually wiped out through the concentrated efforts of the Spanish and Portuguese. McNeill's Law would place the deciding factor squarely on the introduction of deadly diseases and parasites from the mainland to the previously geographically isolated islanders.

This is the likely explanation, as what records still exist show numerous deaths by disease on the islands and a declining birth-rate, leading eventually to the almost complete end of the Guanches as a race.

Other instances include the devastation of the Incas by smallpox.