According to Guns, Germs, and Steel, what protected New Guineans from losing to invading Europeans who sought out lands to conquer and settle?

The answer
to this can be found in Chapter 15.  There, Diamond tells us that there were two main reasons
why the New Guineans were protected to some degree.

First, New Guinea was
home to diseases that Europeans could not deal with.  Most importantly, there was
malaria...