Or a magical unicorn! We are just throwing concepts around right?
In all seriousness folks, humans and cheetahs and a few other species, are uniquely susceptible to extinction by disease.
Something bad happened to us a long time ago and thus we can trace our ancestry to a few individuals! This means one individual group of chimpanzees can have more genetic diversity (and thus greater resistance to diseases) then all of humanity.
But if you really want to bet on what the "next big thing" will be, I suggest you put your money on the plain old (and ever new) flu.
Also the "next 1918" flu will not care if we've conquered HIV or not.