I feel like there is something missing here. He is able to suck out abilities from things he infects and possessed. And, he can cause good and bad symptoms, which to be honest doesn’t help this story. Scientists have speculated that symptoms are meant to get rid of bacteria and viruses. There is still more research needed, but the theory is still pretty strong. So, maybe instead of just using symptoms willy nilly, there can be draw backs. Say, he uses 3 symptoms at once, the virus count should drop. This being a draw back to using these symptoms, giving him a large draw back from using his power. And, if he doesn’t use any of these symptoms, the body he’s infected will wither away. After all, a viruses whole purpose is to hijack a body and use it’s cells to create more viruses. So naturally, infected bodies can die when un checked. Giving him a balance here. If he doesn’t use symptoms, the body will just break down, and if he uses the symptoms to much his existence could be erased.