Infection is unfair and nearly impossible to win human wise if.
2)There are not any nice zombies (As in ones that don't attack and instead block other infected for you/let you kill them.)
3)Theypeople who are infected are not idiots. No really, I once spent a whole game hiding right behind the table in the lab.
Infected mode still is fun though in my opinion.
Edit: It was added about 3 weeks ago.