According to Guns, Germs, and Steel what protected New Guineans from losing to invading Europeans?

Diamond says that there were four factors that protected
New Guineans in this way.

• Most importantly,
  there was malaria and some other diseases.  These diseases made it very difficult for
  Europeans to live in the lowlands or to pass through them on the way to the more
  hospitable highlands.


• New Guineans were not decimated by
  European diseases.  This was partly because they had been somewhat exposed to them
  through Indonesia and partly because New Guinea was not widely settled until a time when
  public health practices reduced the spread of
  disease.


• Europeans didn't reach the highlands (because of
  the diseases) where there lots of natives until the 1930s.  By that time, the idea of
  killing native people to get their land was
  unacceptable.


• European crops and animals don't do well in
  New Guinea's climate.  This meant that it was hard for Europeans to establish any sort
  of major settlements.