A company that is considered too important to allow to go bankrupt, and ends up sucking on the lifeblood of the United States (aka its money). Members of zombie companies are permitted to take bonuses of any size, thus contributing to the general blood sucking.
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.