What do you think might have happened had Indians been immune to the germs introduced to them by the Europeans?

Diseases brought by Europeans took a great toll on Native Americans, who had not built up the same immunities as the Europeans. Smallpox was particularly deadly, but diseases such as measles, typhoid, tuberculosis, the common cold, scarlet fever, and whooping cough also took a toll. According to Carl Waldman Atlas of the North American Indian, between a quarter and half of the Native American population was destroyed by disease after first contact with Europeans. In some cases, the numbers were much higher. According to K.E. Nielsen's A Disability History of the United States, more than 90 percent of Florida's Native American population succumbed to disease by 1700.
It's not hard to see that, had the Indians been able to withstand the diseases brought by the Europeans—say, having been exposed to them in their own settings and developing immunity—they would have been better able to resist the Europeans taking their lands. Further, the levels of death caused by disease would have been traumatic, leaving the Natives Americans psychologically stunned. Without the trauma of disease, the native population would have been less psychologically vulnerable to conquest.
A larger native population probably would still have fallen short of the number of Europeans coming to its shore, but the natives would have possessed significant advantages in knowledge of the terrain. One can easily imagine that "manifest destiny" would not have seemed so obvious to the European settlers. Having more threat of warfare from a larger population of Native Americans would probably have led the Europeans to be more careful about honoring treaties and more willing to negotiate with the natives in good faith. It also would have meant, however, being faced with fiercer battling for each hunk of territory. The Europeans would very likely have managed to be satisfied with less land and a more respectful coexistent with their Indian neighbors. Wars would have been more costly, and European governments thus less likely to want to engage in them. The North American continent today might look more like a patchwork of native and former European nations rather than an English-speaking monolith.