Rempaths were parasites usually from the Rafirax system. They could be transmitted via internet, and infected hosts via the technology they used. They were then carried in the blood where they feed off emotional energy and multiply.
They induced an anger in their hosts which grew, eventually resulting in violent rages followed by a fatal heart attack.
Gully was using Earth as a Rempath farm in order to take revenge on the Tenth Doctor and to sell as weapons[statement unclear]. (PROSE: In the Blood)