In order to explain what a zombie virus is, you must first explain zombies. Zombies are dead or mentally disabled creatures (usually human) which have lost all signs of self-control. Many people have speculated what diseases can cause zombies to be created. The most logical idea is a mutated rabies virus. Such a disease would need to only affect the brain and possisbly degenerate the skin somewhat. If a rabies strand could mutate into a disease which would keep its potency but make the effects last longer then one could potnentionally have a zombie virus.
Scientist: It seems to be a mutated form of rabies which has overpowered his higher levels of thinking. In essence, a zombie virus.