Animated dead tissues from a virus - impossible.
A rage-like infection like what we see in 28 days later - highly possible.
Two contenders:
Creutzfeldt-Jakob disease (CJD) AKA Mad Cow Disease.
It needs to mutate a bit more to become contagious enough to spread, further it has to specialize its attack more on higher brain function and leave the mobility areas of the brain alone.
However a list of its current symptoms:
Blurred vision (sometimes)
Changes in gait (walking)
Confusion, disorientation
Dementia that occurs over a few weeks or months
Hallucinations
Lack of coordination (for example, stumbling and falling)
Muscle stiffness
Muscle twitching
Myoclonic jerks or seizures
Nervous, jumpy feelings
Personality changes
Sleepiness
Speech impairment
Source:
http://www.nlm.nih.gov/medlineplus/ency/...000788.htm
Currently it is not very contagious from human to human, you can be bit by a person with the disease and not be infected. Eating the flesh of the infected on the other hand...
Rabies.
Rabies is the scariest one because it is highly contagious (through bites) and has been known to pass airborne under the right conditions. Its ability to target the higher brain functions in humans while leaving them mobile long enough to spread the disease makes it more of a risk than CJD.
On top of it, rabies causes the victim to experience extreme thirst enough to madden the patient to the point where they forget such things like 'human beings are not drinking fountains'.
The upside, is that by the time the patient gets to the point where they are ready to drink from other humans as though they are water fountains, high fever burns out the rest of the brain leaving the patient immobile enough to not be a serious threat.
Rabies makes for other interesting symptoms such as extreme photo-phobia (light sensitivity) So it could be more or less like the disease in "I am Legend".
Symptom of rabies:
slight or partial paralysis
cerebral dysfunction
anxiety
insomnia
confusion
agitation
abnormal behavior
paranoia
terror
hallucinations, progressing to delirium.
Source:
http://en.wikipedia.org/wiki/Rabies
The only thing saving us here is that rabies is nearly always fatal, with only 6 known cases of untreated rabies recovering.
Either one of this diseases could mutate sufficiently to allow infected victims to walk about and spread the disease via bites.
Both causes dementia to the point where the patient will attack others. Both could have long enough life spans per patient for a patient to spread the disease to many others.
Lord help us if either one become airborne. There are no known recoveries from CJD, and only few recoveries from Rabies.