Better idea for double infection is half of the players (or 1/3) start infected, and the others start uninfected. The uninfected have a designated "Doctor" who has an item, a vial or syringe, that can heal the infected.
The doctors aim is too heal all the infected before the time runs out, or him and all of his teammates die. Like in yours, every uninfected that is killed will be respawned as infected, but every infected stays infected.
The only way to become uninfected is via the "Doctor's" syringe. The syringe would have a cool down time, maybe 30 seconds to a minuet, or it could be set by the host as an option. It would be as simple as going up to a zombie and hold down click. While the Doctor is holding down click, both him and his targeted zombie friend will freeze for 10(?) seconds, when the time is up, the infected respawns as uninfected. The Doctor will have a lot of health, enough so if he is caught unawares he can survive on his own, but it is still required for the uninfected to protect him.
Ways to win would be for the infected to kill all the uninfected, including the Doctor. The uninfected would win by healing all the infected. The uninfected can also win if both teams are still intact. If the Doctor dies, neither team wins or loses, the uninfected just have to stay alive.
Of course that is probably too hard so your idea is probably better.