@Conflict Said
Although they are used in fictional contexts, the aforementioned viruses are based on theoretically possible viral agents. For those of you that don´t know what I am referring to, these viruses are the catalysts for the Resident Evil games. They are what turn people in zombies and cause the epidemics of the games.
Now, as with all viruses, the T-Virus uses a particular kind of cell to reproduce, in its case, the cells of the frontal lobe. These are destroyed as the virus consumes them and then something very strange happens. The virus evolves into a new kind, in which it changes the cells it feeds upon to the oxygen rich tissues, namely the muscles in this case.
Why would a virus, which is initially airborne and feeds on a brain cell, evolve into one that targets the muscles in the organism it infects. I thought viruses had only one staple diet per type.
The C-Virus acts in a very similar way. However, it only uses some of the frontal lobe cells to reproduce, not all of them like the T-Virus does. Why would this variation of the virus feed on less of the brain cells it needs?
I think that the science behind the premise of your question is flawed.
In real world viruses change... mutate... evolve... over time.
Let me recommend a book that will cover your questions and then some... is scientifically comprehensive yet a very easy read:
Spillover: Animal Infections and the Next Human Pandemic by David Quammen