How Columbus sickened the New World: Why were native Americans so vulnerable to the diseases European settlers brought with them?

It is often said that in the centuries after Columbus landed in the
New World on 12 October 1492, more native North Americans died each year
from infectious diseases brought by European settlers than were born. They
fell victim to epidemic waves of smallpox, measles, influenza, bubonic plague,
diphtheria, typhus, cholera, scarlet fever, chicken pox, yellow fever, and
whooping cough. Just how many died may never be known. For North America
alone, estimates of native populations in Columbus’s day range from 2 to
18 million. By the end of the 19th century the population had shrunk to
about 530 000.

Staggering losses. But why, asked a perplexed French missionary working
among the Mississippi Valley’s Natchez in the 1700s, should ‘distempers
that are not very fatal in other parts of the world make dreadful ravages
among them’? The answer seems obvious enough: because native Americans had
no immunity to ...