Diseases become "less virulent" by killing everybody who is not resistant to them.
When Europeans came to Americas, the diseases that they carried wiped out a good portion of Native American population:
Those diseases were not virulent in Europe b/c everybody who could die from them already did. But they remained virulent in unexposed populations.
You can try to predict which % of human population has the genes to be immune to your virus, and then use models to predict how the disease will spread and kill people. Interestingly, modern anti-epidemic and quarantine measures will slow the spread of the disease, but also slow the spread of immunity.
But the virus will remain virulent on unexposed population, including your troops and your citizens. Or you can use a virus that your people have immunity to.
Virus that kills its host too fast is hardly a good application, it will not be able to spread. And infected areas will remain will remain dangerous even after all victims are dead.
One feasible idea is a flu-like virus that weakens most people without killing them, and lets them develop immunity as they recover. It will weaken your enemies enough for you to conquer them with conventional weapons, and everybody will be immune to it after a year. It will have to look like a cold, and have long incubation period to let it spread unnoticed. And you can immunize your own troops and citizens against it.