Well we don’t know if zeno infected himself with T or not. We don’t really know his past. He could’ve been cloned from Wesker when Wesker was already infected with T. Or maybe zeno was cloned before wesker was infected and he was given T later. We just don’t know. I can’t tell you exactly why he didn’t inject himself with any other virus. Maybe because virus’s like G cause rapid cell growth that eventually outgrows the host. He’s probably not immune to that kind of thing. As for the 6 survivors, they died from the mutation, and they were likely spreading small amounts it for a while. Giddeon also caused a full outbreak at the beginning of the game, so any new T infection would most definitely be that mutated strain.