"Viruses tend to be fairly specific in what they do and how they do it. To get a full on zombie with all the different features, wouldn't it be much easier to have 5 different viruses all doing their own few things to add to the overall effect?

Some destroy the myelin sheaths of certain parts of the brain that that allow neurons to pass on information. This could disable our judgment and ability to reason (parts of prefrontal cortex). Some alter the Hypothalamus to produce hormones that alter the blood pressure, hunger levels, and sleep duration. Some virus with a rabies like anger-aggression could occur along with a bacteria that causes excessive sneezing, further spreading the sickness and making it somewhat air born.

Possibly the zombie virus could be a symbiotic relationship between multiple viruses and even bacteria. "