Put these two ideas together, and what if a zombie bite infects a human with a similar type of germ? The best vector for this should be the inefficient zombie. It bites and infects a person, who escapes. The germ is not transmissible to other people, but the infected person produces some sort of pheromone or whatnot that causes people he comes in close contact with to seek out the zombies. This allows the inefficient zombies to have more chances to attack and either eat the brians of or infect more people. Eventually, the infected do become zombies, and the slow change also allows the zombie disease to travel where pure zombies couldn't (airlines, trains, busses, government buildings, etc.).
Too bad neither of the sites have a place to leave a proper comment. :( I started this as a comment to dinosaurcomics, but thought it better to spread the word of the mind-control zombie germs!! Spread the word!!!