St Roch was a French nobleman who lived in the 14th century when the plague was in Europe.  During his travels, he came upon a town in Italy suffering from the disease.  St Roch devoted himself to helping the plague-stricken curing them by making the sign of the cross.  Whenever he visited a city suffering from the plague, it disappeared before his miraculous powers. Eventually he contracted the disease, and he retreated to the forest to die.  He was befriended by a dog, who brought him food, and eventually he recovered.

Time and time again St Roch was invoked during plagues, and this is why he is considered the patron against plagues and epidemics.