Dutch hit on something earlier that I feel needs more attention. Everybody is already infected, symptoms only manifest themselves once you die.
Shane and Rick discussed the two security guards that they killed not having any wound marks on their bodies and finally decided they must have been infected through a scratch. All of the empty food cans and food containers in the gated area. The fact that there were so many zombies in a gated area. It's like these people holed up in what they considered a safe area locked away from zombies and then started to die of starvation thereby turning into zombies. All of the zombies in the gated area were some of the fastest and "healthiest" zombies yet.
Too add, noway you could cut yourself to attract zombies and then kill them in such a gory manner and not have some of the infected goo enter your system. Look at Rick and Shane when they were driving back to the farm. Granted, some of it was their own blood but lots of it was walker goo.
The farm is one of the best places to be. Secluded enough to fly under the radar but open enough to be able to run all while producing enough food to sustain human life. Everyone saying that they'd hole up in a prison or secure compound is likely signing their own death certificate.
