We all know that Rabies has a 99% Mortality rate. We know that once the first symptoms of rabies have presented, the patient is beyond any sort of treatment.
We know that if treated for rabies early, within the first 48 hours, it can be treated.
We also know that the average lifespan of someone infected with Rabies, is roughly 7-10 days.

But what if we could genetically alter the virus? What if the virus were altered in such a way, wherein the host -- (infected persons), live on average 2-3 months before dying from the Virus?

Is this even possible? What sorts of Viruses can be altered and which can't?