Nature has devised some ingenious and horrifying ways to spread germs. Take measles, for example, which is spread by coughing and sneezing. It's a disease so contagious that around 90 percent of people who come into contact with an infected person will also become ill [source: CDC]. It's an insidious virus, too, as it can live for around two hours outside the body, hanging in the air, just waiting for a hapless person to inhale it and start the replication process anew.
Then there are zombies, which have to bite people to spread their contamination. There are an array of issues with this sort of propagation, starting with the fact that it's terribly inefficient.
First, the zombie has to somehow grab a person long enough to inflict a bite. This is a tough proposition for a creature that may be missing an arm or leg, making it challenging to chase down terrified prey. Second, the act of biting consumes enormous amounts of time and energy, two things that rotting zombies are not really known for. And three, biting requires close physical contact with a victim. In a time when survivors will be ever vigilant and hard to find, those cold and slow zombies will be hard pressed to press flesh with any of their warm-blooded cousins.