That's what the Rash does. It mutates people into trolls.

...I should specify that we just call them 'trolls', they're not like the trolls of legend or anything. Trolls didn't exist in my world until the Rash appeared.

They're really dangerous too! Some of them can move unnaturally fast, and all they do is bite people and spread the virus. And, er, eat people too, sometimes.