Hi!
Suppose that when the Europeans first set foot in the Americas, the European colonists suffer the same "virgin soil" casualty rates as the Native Americans did in OTL. IIRC, most of the diseases which the Europeans contracted from the Native Americans were relatively benign. But that could have just been luck.
In this scenario, assume that the Native Americans who contact the Europeans suffer the same 90% casualty rate as they do IOTL for noncontagious diseases. The Europeans do a little better and suffer 75% casualty rates. This is in addition to any casualties the colonists take from the environment and simply trying to settle the land.
Furthermore, assume that both groups have to deal with contagious diseases as well. The contagious diseases are less lethal (since the victim has to be survive long enough to spread it), so they have 60% casualty rates among the Native Americans and 50% casualty rates among the Europeans.
Imagine what would happen if a disease-ridden group of explorers returns home to Spain, England, Holland, or Portugal and started spreading the Native American diseases throughout the New World. Will the meek inherit the earth as the great powers suddenly get sick? You saw what happened to the New World when the Native Americans fell ill. Imagine that happening to the Old World as well.
ACG