You can win as crewmate as long as you don't get infected with the thing or alien, but that only happens to people who have breached suits on I mean they are in space in two of the levels and in the third they are high up in the air, and no if the alien was air born it wouldn't work if it had a fliter system in there tanks, and with each person a infected kills another infected will rise, with only long term ways of getting rid that require setup but judging that for these infected to be useful their target has to be alone from how in the game they work you don't see them killing everyone which could be lore or gameplay but either way im counting it I think that as long as your suit doesn't get breached you can live from the infection and can for sure live out the imposters and actually win