Get the dirty cage ... play with the rodent .. get bit. I had two dirty cages so had my sim get bit twice. The disease is by no means quick also. You'll get bit then it takes like a day to get symptoms and then goes through other phases. You have plenty of time to research a cure and buy it or go to the vet. I just bought the cure for my sim on the computer but he's still not contagious I don't think.. I know I could look it up but it's fun just watching. Lol I don't think he'll spread it well but I plan on having a club gathering and bring in all the victims.. hopefully he'll pass it on then take the cure and I'll see what happens to the locked in sims. Or I could send them home and switch households and see what happens. I want to keep my main sim alive.