Description
I added mosquitos to minecraft. They might give you malaria, they might not.
Mosquitos:
Spawn in jungles, swamps, and lush caves at night
They have one health and only deal one damage on each hit, so they aren't that big of a deal... unless they give you malaria
Malaria:
Getting bit by a mosquito gives you about a 10% chance to get malaria
Malaria takes a while to show symptoms, so you won't know if you got it until about 7 days after being bit
Without treatment, malaria reduces your attack damage, jump strength, and movement speed, and will kill you in about 3 days.
Unfortunately there is no current treatment in such a primitive world, so your best bet is to not get malaria.