Malaria : Mosquito :: Plague : ? Which word completes the analogy?

The relation is a disease : the creature that carries it to people. Malaria travels by the mosquito; plague travels by the rat and the fleas it harbours. Every distractor carries some other illness, which is what makes the choice feel open — the housefly settles on food and spreads typhoid and cholera, the dog passes rabies in its bite and the pig carries tapeworm.