Xilobytes were a type of infectious, swarming insects. They started life as nearly microscopic larvae, which would find themselves a host. Once the host was infected, the Xilobytes would start to feed on its flesh. The host would feel weaker, with mild, flu-like symptoms. The Xilobytes would grow and feed, eventually eating the host to the bones. The only known method of getting rid of them was to physically pull them off.
The Hokrala Corp tried to kill Jack Harkness by sending a writ infected with Xilobytes. Instead, Ianto Jones handled the writ, infecting him. Though he nearly died from the infection, a Vortex Dweller changed time and removed the Xilobytes. (PROSE: The Undertaker's Gift)