Wounding often creates a larger logistical problem for the recipient of an attack. Perhaps 3% is an optimal number for this biological if it is indeed a weapon.
There's no wounds from viruses, most recover completely. Much higher lethality limits the spread and lethality is not easy to figure out so 1 to 10% is probably just a ball park estimate for an engineered weapon.
Yes, it is more difficult to deal with sick and wounded than expired.