Loss

Jackie Morse Kessler’s Riders of the Apocalypse series follows teens who are transformed into the Four Horsemen of the  Apocalypse.

The third book in the Riders of the Apocalypse series, Loss, is about a bullied teenager who's tricked into becoming Pestilence, a Rider of the Apocalypse, and finds himself with the power to infect people with diseases.

Fifteen-year-old Billy Ballard is the kid that everyone picks on. But things change
drastically when Death tells Billy he must stand in as Pestilence, the White Rider of
the Apocalypse. Now armed with a Bow that allows him to strike with disease from
a distance, Billy lashes out at his tormentors...and accidentally causes an outbreak of
meningitis. Horrified by his actions, Billy begs Death to take back the Bow. For that to
happen, says Death, Billy must track down the real White Rider, and stop him from
unleashing something awful on humanity—something that could make the Black
Plague look like a summer cold. Does one bullied teenager have the strength to stand
his ground—and the courage to save the world?