The global transfer of disease in the pre-modern world helped in the colonization of the Americas. The reason was that the native Americans were not immune to the diseases that the European settlers brought with them. The Europeans were to a certain extent immune to the effects of diseases like smallpox due to centuries of exposure, but the native Americans had no such defence against this disease, as they were isolated from diseases native to the old world.

At times, settlers deliberately practised biological warfare on the natives by giving items laced with smallpox germs as ‘gifts of friendship’. The disease was far more effective in wiping out entire tribes and communities without having to resort to firearms.