[Suicide Junkie]

As for 'types' of bio-weapons, you could have 'poisons' that are described as either chemical or biological that destroy immediately upon use but don't continue from turn to turn or linger after the population is gone, and you could have 'plagues' that are contagious and actually linger after the victims are gone, making the place unliveable for whatever they infect for years of game time.

Those could be plagues with a virulence of zero... They'll incapacitate and kill the people they've already hit, but won't spread at all.
If left untreated the incapacitated people would start to die and the unaffected people would become incapacitated, but no uninfected people would get the plague unless a ship was still up there dropping bombs.

Eg: 100 M people hit out of 500M on the planet, 10% fatality rate, 50% incapacitation, and no virulence.
turn 1: 10M die, 50M incapacitated, 40M infected but ok.
turn 2: 90M left... 9M die, 45M incap, 36M infected but ok.
turn 3: 81M left... 8M die, 40M incap, 33M infected but ok.

Edit: Oh yes, plagues should travel between planets unless you implement quarantine which is the same as your planet being blockaded! All trade stops and whatever the planet produces is not available to your empire.

Ooh, yeah. I forgot to add that ships not carrying population should count as 1M infected people after visiting a plague world and travelling to another planet.
Given a virulence factor of 2.0, you'd end up with 1M infected on the planet at turn 1, then 3M, 7M, 15M. (2,4,8,16 minus the 1 that are from the ship)

Minimum new infections for non-zero virulence would be 1M per turn, just like how population growth works in SE4.