I've read in some answers on here that this is incorrect and that much of the impact of disease was from changes in practices as a response to Europeans, such as indigenous populations being resettled in higher density, unsanitary centres.
But the Wikipedia article on the depopulation of the Americas says that something like 80% to upwards of 90% of the indigenous population were eliminated, quickly, and that the diseases spread ahead of the settlers.
A Harvard study looking at tree rings and using Lidar to map abandoned villages determined when these villages were abandoned, and it was roughly a century after European contact. One of the authors said his hunch was that it was the introduction of European livestock. This doesn't match with either theory, the one about rapid depopulation or the one about it being European settlers forcing indigenous peoples to change their practices, but the general idea from this study is that the depopulation was more gradual.
This doesn't match with either theory, the one about rapid depopulation or the one about it being European settlers forcing indigenous peoples to change their practices, but the general idea from this study is that the depopulation was more gradual.
In reality, it's a combination of all three scenarios. There's no one-size-fits-all description of how Native American communities were depopulated over the centuries. Some areas, like eastern Massachusetts did suffer rapid depopulation due to disease immediately prior to European colonization of the area - though not prior to European contact more generally. The Ohio Valley at large, was depopulated between the 1630s and the Great Peace of 1701, after two destabilizing waves of epidemics (the 1633 Smallpox epidemic and the 1635 Influenza epidemic) and the subsequent Beaver Wars, repopulated by various Native nations (a mix of exiled nations returning home - the Miami and Shawnee, for example - and "foreign" nations moving into the claimed territory - the Iroquois and Lenape, for example) all filling the voids left by the collapse and restructuring of Native societies in the area. But the epidemics weren't done with the area. Another major outbreak of smallpox occurred in the mid-1700s, during the French-and-Indian War and Pontiac's War that followed. In the 1790s, American settlers started entering the region in earnest, as the government was doling out the Ohio Country to pay of the veterans from the Revolution. This prompted many small skirmishes, two major wars, and numerous exploitative land deals until the Indian Removal Act pushed the remaining Native communities out the region in the 1830s.
A broad scale, continent-wide explanation is always going to be a rough sketch. It's always better to look at specific regions whenever you can, because different areas are hit by the effects of colonialism at different times, at different rates, and in different ways.