I think you go through cycles when it comes to contracting viruses. When I was at college I was always home off sick and I always lost my voice as well. That never happens now. Maybe if you are exposed to viruses often enough, you get immunity. The only thing you can do is eat a healthy, varied diet. Get lots of sleep and don`t get too stresses (been proved to affect immune system) I`ve always believed that if I think I`m going down with something I need to rest. I would never partake in exercise because I would be diverting my energy from fighting the virus