I'm not sure how he got a virus either but I sure hope he feels better soon. One time I took all 3 cats to the vet at the same time. Well Storm came down with a UPI and so he then gave it to the other 2 cats as well. They are all indoor only cats and I told the vet that the only place they had been was at his office. The vets response was that there weren't any URI's going around at that time and he hadn't seen any cases in his office either. He also said that indoor only cats usually have weaker immune systems and that stress can sometimes make their immune systems weaker. I also know that many viruses are air born so if a window is open then a virus can infect an indoor cat. You'll probably never know the answer to why Loki suddenly got sick.