What caused the Amerinidians population to decline in the 16th and 17th century?

I think that because of the disease and bad living conditions killed many of the Africans and Europeans. The Africans had a very high mortality rate and disease was spread quickly because there was no room in the ships so there could be one infected person on the ships and everyone he was next to for the trip (about 5-7 months) could catch it and the person next to the newly infected person could catch it, etc. About 15% of the Africans died on the ships coming to the New World.