If everything else were to remain the same; keeping the infected alive longer wouldn't necessarily do much at all. The trouble comes when that person spreads the virus to someone else, perhaps with a compromised immune system. If the virus load in the newly infected person is the product of that portion of the original virus which wasn't killed by the "99% effective" methods of killing them, then that victim now has a more robust form of the virus, likely harder to treat.

But not necessarily more virulent.