V-Ko and Vi can only cure SSTDs, not give you immunity to them (and same with Bianca). Doctor Lash will give you SSTD immunity if you pay for it, which will also halve your current fertility and virility. Tika (the Steele Tech med-bot mentioned in the above post) will give it to you for free, with no side effects. Doctor Badger won't, probably because she invented one of the SSTDs in the first place.