Why were europeans less vulnerable to most diseases than native americans?
a. they had built up immunities.
b. they never came into contact with them.
c. native americans were less hygienic than europeans.
d. native americans did not practice medicine like europeans.

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.