Why did diseases kill so many Native Americans?

When people from Europe and Africa came to America, they brought their diseases with them, which proved to be devastating to the Native Americans.   Diseases like smallpox and measles can be very dangerous to anybody, but they were especially dangerous to the Native Americans because they had no natural immunity to these diseases.   When your body first is infected with a disease, its natural defenses begin to kill it off.   If your body is successful in fighting the disease off, then your body is ready for the next time you get the same disease.    You might have full immunity from the disease and it will not affect you at all, or you might have partial immunity and you will get sick, but not die.  

Even before recorded history, diseases like smallpox and measles were killing off the people of Africa and Europe.   They killed off huge numbers of people, but the survivors were the ones who had some immunity to the diseases.   The survivors passed on their immunity, the ability to fight the disease, to their descendants.   It was those descendants who traveled to America.   When Europeans became sick, they did not die, but they passed the diseases on to the Native Americans.    Those diseases [and others too] began the same process of killing off those with no natural immunity as it had done in Europe and Africa.   The diseases may have killed off 90% of the Native Americans.   Those Native Americans that are living today are the ones who have some immunity to the diseases.    The huge number of Native Americans who died was the result of new diseases being introduced to a group of people who had no natural immunity to those diseases.