This has to be the most random mission I've come across. The "strong or elite" theory might help but not always.
Some of our guild have spent (literally) hours trying to get infected, hacking away at every rakghoul they come across. I think 3.5 hours was our guild's record when they finally got credit two zones over.
Our first time through we noticed that the timer reset itself after the initial 45minutes ran out. Might have changed since then.
I just don't get this. I've run this mission with 6 toons on live and with another 5 heading back to beta. I've never failed to get infected within 1 minute.
I'm not trying to say anything against you, I just think people (in general) over think this mission. They run all round and don't get infected and wonder why. It's really not that hard. Run right up the pipe in the north, leave your companion on passive, and engage the red rakgoul by the tree. Stand there and let him hit you (occasionally, I've had to use a med pack while he's hitting me) until you get infected. It's worked for me every time.