Sounds like a viral infection if it is spreading and killing the rats.

Mycoplasma is quite literally in every single rat. It flares up when the rats immune system is stressed. Mycoplasma is usually not a problem, but as the rat's age they will slowly succumb to the effects of the lowered immune system, leaving the door open for a myco flare up. Young rats do not show the effects as much as an older rat. The sneezing, poryphrin staining around the nose and eyes, these are all usually from an irritated respiratory system (no ventilation, build up of ammonia, dusty bedding, pine or cedar) further irritated by the myco. (lead to respiratory distress, etc)

Another idea, is if you had any other rodents in the same room as your breeding colony. Quarantine is of the upmost importance because the transmittable viruses and bacteria from foreign rats will quite literally kill your whole colony with efficient speed.

It could quite literally be any number of illnesses. I'm sure Becky will have better and more detailed ideas.