Why did so many native americans die after coming into contact with the europeans

The europeans brought infectuous diseses such as the bubonic plague, chicken pox, pneumonic plague, cholera, diphtheria, influenza, measles, scarlet fever, smallpox, typhus, tuberculosis, and whooping cough. The diseases introduced in the Americas by the Europeans were crowd diseases: that is, individuals who have once contracted the disease and survived become immune to the disease.