Rinderpest

CIC is a list of trivia at Strategypage and includes this:

Rinderpest, an ancient animal virus that swept across sub-Saharan Africa in the late nineteenth century, devastating cattle, and thus facilitating European imperial expansion in many areas, was accidentally introduced to that continent in 1887, when infected cattle from India were landed at Massua in Eritrea to feed Italian troops on colonial service.

Of course, the reason for this was that, by decimating the wild beasts that allowed the tsetse fly to live, it allowed European cattle to thrive and allowed people to live without the worry of sleeping sickeness.