Air system - go to the glass enclosed room. At the extreme right behind a big tank is a stand. use the gas canister. Adjust the stand beside it to 1/2 load. Go out the enclosure - wear gas mask. adjust the stand to full power.
Virus is at the professor's room. I think far left of the second row of rooms.
Be sure to scan the General at the enclosure in the room with the giant map.
Have fun with the ending!