Question

Why were many native Americans vulnerable to European diseases

Answer 1

I'm just ,a king a good guess but the natives did not know that Christopher Columbus had been carried the small pox with him so many of the natives died of the disease

Answer 2

The correct answer is that they have built up immunities!
The illnesses which lead to the death of  many native Americans have been present in the old world for a long time already that many people have already died from them. Those that didn't die were immune and passed on this immunity to their children.