oh no

I just found Silas suddenly sitting somewhere (he seemed fine earlier), with some sort of slimy, slightly foamy stuff coming out of his mouth. And he isn't even grooming it away. I've never seen anything like that with a rat (they don't vomit or anything). He seemed fine earlier, not even an hour ago when I filled their bowls with food and gave them some treats. And then suddenly he's like this! I hope he'll be okay until I can get him to the vet tomorrow morning, and that whatever is afflicting him is not contagious, or that the food is somehow contaminated with something and they are all going to die (I have given them from this box the last few days already though). Sick rats are bad enough, but a rat sick with something you've never seen before and can't gauge is ten times worse. Please cross your fingers for the poor guy. He's not even a year old. And why do these things always happen in the middle of the night?