[spoiler]it is a virus. everyone is already infected and a carrier. it takes death to make someone turn. bites cause sepsis and massive bacterial infection due to the fact zombies are decaying flesh with all kinds of decomposing bacteria [think komodo dragon/monitor lizard spit], and even a normal mouth has many kinds of bacteria.. [/spoiler] well if you haven't read the comics, i suggest getting the compendiums and novellas...spoilered for those who haven't.