I've heard a few people joke about a zombie apocalypse happening sometime in the future, but others who have said this have been serious about the possibilities of it happening. They say it will most likely happen due to doctors over-prescribing dangerous drugs to patients, as well as people being on street drugs.
Having seen some videos of the behavior of a few severe drug addicts, it appeared that these people were actually turning into zombie-like creatures, making strange movements and sounds, and tearing off their clothes. It is a pitiful sight, but I'm sure those addicts didn't think they'd ever be reduced to acting like something from another world like that.
There are way too many available drugs in the world, and no matter how much the government claims that something is being done about the problem, it doesn't look like it will ever go away, because a lot of addicts are self-medicating a mental illness that they have and are trying to deal with day by day, and they can't afford rehab or any other programs, so they just stick with the harsher street drugs.