Zaire

An ebola outbreak, nasty business.

I've had a morbid interest in ebola since reading "The Hot Zone" in my teens.
It's such an ancient evil from the bowels of the deep jungle in the congo, its like something hollywood would have created.
Compared to HIV/AIDS which is a sophisticated and complex virus, with such specialised ways of hiding and replicating, and specific attack mechanisms, Ebola is a brute force, total annihilation type, which just breaks down tissue and internal organs to a primordial soup, like some sort of rapid de-evolution.
Plus its incurable, It's one of humans biggest biological threats.