I'm afraid I can't answer that. It is said that only a Buddha has the capacity to see the ins and outs of Kamma. But certainly I have seen animals that help other beings, being able to make some positive Kamma.
As far as a mosquito biting someone, it probably doesn't know it has a disease. It's just acting in accordance with its nature. It wants to reproduce and that urge is very strong in most beings, but I'm afraid I can't tell you the results of that Kamma. That would take a Buddha's mind.