More often than not, a virus is what causes the zombie outbreak. It's a good candidate for this illness because viruses are hard to fight, they act quickly, and they mutate fairly rapidly (look at the flu virus, it mutates every year). Viruses have the capability of causing mass casualties, and throughout history, they have. They are scary.
I did a whole chapter in my nonfiction zombie book about viruses, and it was a lot of fun to research. There were parts of it that were frightening, such as Ebola viruses, but it was also intriguing to read about how we have evolved alongside each other. In the process, humans developed an immune system to keep them safe, and viruses keep finding ways to get through that system.
I also got to talk to a guy from the CDC about disease containment. It was fascinating!
Some films that portray zombies coming from viruses: World War Z, 28 Days Later, 28 Weeks Later, Resident Evil