The global transfer of disease was a powerful and unintentional weapon that greatly aided the European colonisation of the Americas. The native inhabitants of the Americas had been isolated from the rest of the world for millions of years and therefore had no immunity to common diseases from Europe and Asia. When European colonisers arrived, they brought with them germs of diseases like smallpox, measles, and influenza. Smallpox, in particular, proved devastating. It spread deep into the continent, often ahead of the Europeans themselves, and wiped out vast portions—in some cases up to 90%—of the indigenous population. This demographic collapse decimated entire communities, making it easy for the Spanish and Portuguese to conquer and occupy the land with very little military resistance.