Yellow Pill

A man accidentally discovers a pill that stops the spread of the zombie virus, but it only works if you haven't already started showing symptoms, once you have symptoms you're done. It won't bring back anyone who already turned. It will stop the spread if you take it early enough, and it will keep you from getting infected if you take it before infection. The only problem is the pill is only carried by a lesser known discount grocery store chain, so finding more won't be easy.