We're not in the middle of a zombie plague right now, but there are certain diseases out there that are mutating in strange ways; seems like they could possibly lead to a zombie sickness. A couple most compelling ones are prion diseases. A prion is a mutated protein that when it gets in your brain it mutates all the other proteins in your brain.
Mad Cow disease is a prion disease. Mad Cow disease in people, which is called CJD or Creutzfeldt-Jakob disease. New indications with CJD is that the symptoms are changing and it's becoming more easily passed from one person to another, and from animal-to-human, so much so, that some of the new symptoms include in late stages, violent aggression. Some experts have recommended that if someone starts showing advanced signs of variant CJD, vCJD, you should lock them in a room, let them go violently insane until they die. If they attack you and bite you, they could potentially spread the sickness to you, and then you will go violently insane and spread it on. That's exactly the premise of 'Zombieland'.
Another obvious candidate is rabies. The Zombie Research Study has interviewed several virologists that have suggested that it would not be impossible to combine a highly-deadly strain of rabies with a highly-deadly strain of the flu; mix in few other viruses to cause this super virus to be faster-acting, and essentially, create a super rabies that causes people to go violently insane and attack one another.
We may not be in the middle of a zombie attack right now, but in my opinion, all roads point to zombies.