Red Worm Fever

As if the very wilds of Aranel were pushing back against the colonizers, the Raivanni and Derathaini have proven highly succeptible to an illness they've named Red Worm Fever, so called due to the red skin lesions which appear on the affected. Other symptoms include high fever, nausea, delerium, blurred vision, and difficulty breathing. The onset of these symptoms is exteremly rapid, with the patient debilitated within hours of the first lesion becoming visible.
This illness is actually carried by a small insect native to Aranel, the kaq'ropo. This small flying creature lays its eggs in the skin of mammalian hosts, which hatch into parasitic larvae. They feed on the host's tissues and fluids before chewing through the skin and flying away as new kaq'ropo. This is most common is beasts, but has been known to happen to sentient beings as well.
The native Aranellans seem more resistant, usually being able to overcome the worse effects of the fever in a matter of days, but Raivanni and especially Derathaini have been laid low for weeks and even died. Theories abound as to the reason they are affected so much more than locals, ranging from diet, to physical features, to even conspiracies of an intentional curse. Those few Aranellans who've commented on it claim it's simple nature, with different peoples being imacted differently, but of course most colonists find this inadequate.
With ongoing hostilities between the Aranellans and the colonists it seems likely this will be further catalyst for unearned distrust and hatred.