Mosquitos can often trnsmit viruses from one organism to another acting as

the act as vectors of said viruses

Mosquitoes can often transmit viruses from one organism to acting as vectors.

Explanation:

Certain microorganisms have their life cycle that is split on 2 different organisms, these microbes need the contact of both the organisms to complete their life cycle a classic example for such a type of microbe is the Plasmodium vivax, which is nothing but the malarial parasite.

This malarial parasite has its vegetative phase in a mosquito namely the female Anopheles mosquito and the reproductive phase in a human being. In such case a mosquito that is infected keeps the parasite within it and enables it's multiplication, when it bites an healthy individual it transfers the organism into the blood stream of an healthy individual.