If the others have a virus you should be seeing signs...sluggishness, lack of feeding, wet frass, loss of coloration, etc. Once infected, the virus replicates rapidly and can kill the larvae within a few days I believe.
You could also be seeing a bacterial infection...here are a couple of photos:
http://faq.gardenweb.com/faq/lists/butt ... 31715.html
Either way, it is a good idea to wash leaves you feed to your cats with warm soapy water (rinse well!) and keep rearing chambers clean and dry. Isolating your caterpillars is another way to prevent transmission of any disease. If you suspect infection, sterilizing your containers/chambers with a 20% bleach solution (again, rinsed well) is crucial.