Aboriginals are in essence a representation of the original and earliest 'Caucasian', before they moved east and then south - 'away' from Europe.

Makes you wonder what it was that drove a large group of people to undertake such a continual (wouldn't have happened in just one generation) journey from Europe, across part of the Middle-East, through India, down through South East Asia and across the Indonesian Islands, Melanesia and finally into Australia!
The fact that there were already more earlier species of Hominids existing in pockets all the way to Flores Island in Indonesia, let alone other 'Sapien' tribes that provided other 'genetic' links when inter-breeding came into it.

The America's were populated by a single 'Tribe' from far northern Siberia - that just shot off across the land-bridge into North America. Across a very harsh and frozen environment during an ice age. That single Tribe ended up populating two continents.
...could explain their weak genetic immunity? 

But in truth - the strongest 'Gene' pool is the African one (the 'Black' African). Like a Samurai sword folded many times over, the African gene-pool is where all the other races 'originated' from.
Proof is in the San Bushman race, where they have every genetic link that every other race has - but every other races is but a branch to their trunk.

White Race, Brown Race & Yellow Race all came from the Black Man's 'seed'.