you have a fungus growing in your vents or the evaporator. reason for that is that there is water sitting in ther and moist dark areas are the best surface for a fungus.
as you go along try to remember to turn the air of 1 or 2 mitits before u arive at your destination so that the vents temperature rises up to the outside level so that you dont have any water building up inside your vents.
also get a spraybottle of lysol bacterial killer or something like that turn ur air on full blast (feed from outside) open all windiws and spray a good portion of the bottle into the intake (located outside of ur cabine under the windshield by the wipers.(to find out take a pease of paper and but it to that location and see where the air gets sucked in.

do that twice or three times after another (like mon, tue, wed) and u should get rid of it

worked for my other car wasnt an aveo but thats a universal problem i also changed the cabin air filter (dont know if the aveo has one)

Good luck