Actually, there is something called an "HIV superinfection" where a patient acquires one or more additional strains of the virus, potentially accelerating the progression of the disease or complicating treatment due to different drug resistances.James Joseph Emerald said:That doesn't really make sense. If you're HIV positive, and you're exposed to HIV again, you don't become more HIV positive.CheckD3 said:In short, the virus infects everyone, but only a few don't have the immune system to fight it off. When bit by the Walkers, the disease is strengthened, and body can't fight it back. It easily explains why virus started slow and grows and grows.
http://en.wikipedia.org/wiki/HIV_superinfection
Anyway, from what I understand, walker bites cause an incurable, inevitably lethal infection, but I doubt that even the writers have devoted too much thought to whether zombie saliva contains a different strain of the virus or various corpse bacteria, or whether zombies have special glands that concentrate these pathogens in their saliva but not their blood.
Let's face it, the most obvious thing we learned from the CDC episodes is that the writers have never seen the inside of a microbiology lab, and actually think that you can fire a gun inside an active MRI tube.