Native populations in America and the Pacific were devastated by diseases brought by Euroean explorers, but why didn't populations of Europeans in Europe catch these diseases passed on by sailors? And how did colonialists survive initial disease contact at all in the first place?
Europe was part of a wider landmass including Asia and Africa, where diseases passed regularly from one region to another: Europeans thus had a wider range of immunities than peoples of the Americas, who also derived from a far narrower gene pool (perhaps a few hundred original migrants).
Europeans did suffer disease, and some early settlements died out or came close. Syphilis was thought to have been brought back to Europe from the New World in the 1490s, though others have suggested it was already present.