Pistus flies are mosquito-like flying insects that feed on the fluids of living beasts. In recent times they've become an unwitting ally of Phyrexia, as they are capable of slurping up and then spreading the Phyrexian contagion to other creatures. A sufficiently hungry swarm of pistus flies can even take down a dragon—and leave enough traces of Phyrexian oil in their wake to leave you a droplet of poison too.
"Even a nuisance such as the pistus fly has a purpose in our new world." -Glissa