Mosquitoes are animals which spawn in the jungle biome. If encountered, they will infect the player with Yellow Fever which cannot be cured, but will disappear after 35 seconds. Yellow Fever increases player's temperature and reduces the hunger bar to 30% its maximum. Multiple bites from a mosquito swarm will restart the period of sickness, and may cause death via the mosquitos themselves (rather than the Yellow Fever).
Currently, mosquitoes cannot be killed but can be trapped using buildings or avoided like any other animal.