All I can tell you is back when I was first breeding rats I often picked from a feeder bin at a pet store and often saw health problems. These rats are often overcrowded, fighting for access to food and water and come sometimes from sketchy suppliers in the first place.

Better to either buy from a home breeder or even from a good pet store from their pet rat selection. Give the rats a total going over....look for good body weight, good skin and hair condition, dry and clean ears, butts and noses...no red discharge from nose or eyes....no sores on their tail or bottom of their feet....that sort of stuff. Buy young just recently weaned rats, keep them right and they generally will do very well for you.

I agree with Sean that you may want to figure out what's spreading through your colony before you have a whole pile of dead or dying rats. It sounds like they weren't in great condition in the first place and likely have just gotten worse. Double check your own care of them to make sure you are providing good housing, food and water. Rats are very resiliant creatures but can't produce top feeders under the wrong conditions.