You might like some of the answers in our Frequently Asked Questions wiki section dedicated to the impact of disease in the New World. Also check out this post I wrote for another community on the myth of death by disease alone in the Americas. To briefly sum up, while disease did have a tremendous impact on Native American populations after contact, that influence is often overemphasized and divorced from the larger ecological context. The toxic colonial cocktail combined disease, territory displacement, instigated warfare, intentional resource deprivation, slaving raids, and other forms of overt violence to increase Native American mortality and prevent recovery following epidemics. Feel free to dive into those other answers for a more in-depth analysis.