Late 19th century and beginning of 20th century, there was awakening in the Western world both in scientific terms and politically sense.
While primitive societies were in abject poverty and were under colonial Western influence.
There were no human rights charters but human right violation was the vogue in the name of attempt to civilize these primitive people (you should refer to my comments on progress of civilization including alien elsewhere).
West and Mid Africa especially Congo is a very good example under Belgium King Leopoldville.
When a civilization get isolated in an ecological niche, there are certain characteristics built into the system. This changes very little with their population expansion and migration from an unfertile niche to an adjoining fertile niche, not occupied by an alien civilization.
But when a complete foreign or alien civilization encroaches upon these primitive civilizations catastrophe ensues.
First reaction is overt and covert war.
Then it leads to either domination by the aliens or succumb to the primitives.
Numbers invading are small by default but, if they have advanced technology (guns to begin with) there is no defense for the primitives.
This is what happened to Native Americans with Spanish invasions and later by German, French, British and Belgians in African continent.
These invaders bring diseases to the natives which varied from syphilis, to all venereal diseases, to measles, to influenza to aids.
(The sars epidemic is different.
Usually there is no man to man transfer in zoonoses.
It is usually animal to man and it terminates either with demise or of recovery of the human individual.
When these viruses mutate and acquire variants that can transfer infection from man to man, pandemic results.
Pandemics can wipe out entire civilizations.
Pandemics were used by alien migrates to decimate the natives.
The next wave in this design is the coming of volunteer medical personal with putative medications as healers.
When this strategy fails the divine visitors or priests with abundance of money often amassed by ill gotten means replace the medicine sans frontiers.
Viruses can be deadlier than the bullet!