nah i think to become infected, someone who has the infected callsign thingo as being used, stabs you . then you're infected.
people getting nukes pisses me off so much i swear. when they get it, i just leave lol. 2 free for all games in a row someone got it, i only got killed by that guy twice so annoying.
as for the challenge thing, nah never happened to me. but my ps3 has frozen during searching for games.
are you sure its unlimited ammo and not just that scavenger perk? i love that lol. i used to have a class with that, scar-h + grenade launcher, and the grenade launcher as secondary XD