I'm going to post the same thing I've posted in a number of other threads about Native American disease deaths.
The amount of death due to disease is certainly related to a lack of biological resistance due to no deomestic animals, lack of exposure to the same disease in the past, etc. However, this is not the only factor which lead to high death rates. For example, the death rate due to the Black Death in Europe (which people also lacked biological resistance to) was high, but not nearly as high as the post-Columbian epidemics in North America. There are a number of reasons for this:
1. All the epidemics struck at once rather than one at a time. Technological level of the Native peoples won't really impact this, although sustained trade and contact before conquest is attempted could lessen this effect. A higher tech level of Native Americans could lead to them being seen as trading partners rather than potential targets for conquest and conversion.
2. The epidemics were followed (or preceded) by wars of conquest meaning that lifestyles were already being disrupted. An increase in the Natives' tech level would DEFINITELY affect this one, as Natives with gunpowder and steel could mount much better military resistance to the Europeans.
3. The pre-Columbian Native Americans lacked "cultural resistance" to disease. Cultural resistance is a collection of cultural practices which create a more disease-resistant population. They include things like practices of quarantining (leper colonies, etc.), high birthrates to overcome childhood mortality due to disease (the Native Americans had much lower birthrates than contemporary Europeans), political traditions of calling for solidarity in times of disease (some Native Americans responded to disease by waging "Mourning Wars" on their nieghbours), etc. This sort of "cultural resistance" usually evolves slowly in cultures exposed to repeated epidemics. If you want to avoid massive die-offs due to post-Columbian epidemics, you have to increase cultural resistance to disease, and I'm not sure how reasonable it is to do so without introducing some indigenous epidemic diseases to give an incentive to make a cultural change. Increasing material technology on its own won't do anything about this.