Plague

Plague when it appears usually appears as a male rider on a green horse or an emaciated pox covered woman of indeterminate age.
He or she is possibly the most hated of the four horsemen if not the most feared.
Many believe that Plague is a reincarnation of Nurgle.  Some whisper that Famine may be a reincarnation of the Great Horned Rat.
Plagues minions often take the form of flies or rats.