Xilobyte

Xilobytes were a type of infectious, swarming insects. They started life as nearly microscopic larvae, which would find themselves a host. Once infected, the Xilobytes would start to feed on the host's flesh. Due to an anaesthetic, the host would only feel weaker with mild flu-like symptoms. Over time, the Xilobytes would grow and feed, eventually eating the host to their bones. They only known method of getting rid of them was physically pulling them off,

The Hokrala Corp tried to kill Jack Harkness by sending a writ infected with Xilobyte. Instead, Ianto Jones handled the writ, leading to him being infected instead. Though he nearly died from the infection, a Vortex Dweller changed time and removed the Xilobytes. (TWN: The Undertaker's Gift)