Common mosquito half full of blood after biting a leg (feeding) (Sequence 2 of 3). Only the female mosquito feeds on blood to nourish egg development. Otherwise male and female masoquitos feed on plant nectar. Mosquitos can carry diseases such as malaria, denque fever, yellow fever and the West Nile virus. Disease carrying insects are known as vectors. After a mosquito has bitten its host, it injects saliva which is an antigoagulant allowing the insect to "drink" the blood without it clotting. It is the reaction to the saliva that causes the swelling and itching of the bite.