Actually...
It's possible for a disease to be too deadly to be dangerous. Something that kills fifty percent of its victims rapidly sometimes acts as almost a "firebreak" in epidemics.
Diseases that take a long time to act after exposure but then have a high fatality rate are probably the worst.
Eri made a good point on tranmission. Best case scenario is when a virus "jumps," is mostly benign, yet can confer some immunity to worse viruses of the same type later on.