If it's a polyhedrosis virus, you have to get rid of the chrysalides (not near your milkweeds!) and move the other cats to a clean dry container, with all new food, or it will spread. Polyhedrosis viruses are used for biological caterpillar control instead of spraying, like for getting rid of cabbage loopers or gypsy moths. The caterpillars that ingest them will eventually turn brown with their guts liquefied, making it easy for the virus to spread. I read that it's contagious enough that an infected adult male can transmit it to its chosen female, who can transmit it to her eggs.
You can't do anything for the cats that may already have eaten contaminated food or otherwise come in contact with it, they will all get those brown splotches (something about the viruses making crystalline structures out of the caterpillar's body) and die, either as cats or in the chrysalis. The few articles I've read about this indicate that it's always fatal and that new cats can also get infected just by coming into contact with the leavings of an infected one.
Univ. of Minnesota has a site with some pictures of polyhedrosis infection, they call it "black death."
Monarchlab Parasite Page