Why don't the Infected attack each other?
The infected seem to be able to live in groups,without inter-infected violence. Also, while it isn't shown whether they eat and drink, and some clearly starve to death, it would seem that at least hydrating themselves happens, because they would die in less than the normal human 3 days, because they vomit the blood and black bile all the time.
This being the case, if, in fact they can eat and hydrate themselves, and they don't kill each other, and seem to be able to live in groups of mixed sex, as implied by the attack scene at the blockade, they could become a self perpetuating species.