When Native Americans First Encountered Europeans, What Led to the European

When Native Americans first encountered Europeans, what led to the European diseases being so deadly?

A) Native Americans had been struggling with basic survival.
B) The diets of most Native Americans lacked meat, so they had no consistent amounts of protein.
C) Most Europeans spread the diseases on purpose.
D) The Native Americans had no tribal doctors or healers.
E) Centuries of continental isolation meant the Native Americans had no immunity.