Always keep your team members well stocked with arms and ammo, even if they are infected. It is easier to kill an infected member and take all his stuff then it is to backtrack trying to find more ammo.
Don't waste your medkits that determine DNA. If a team member is infected it will manifest itself in due time. Infact it seems to be scripted. I did a test on a member and it showed nothing. Then 5 minutes later he was a nightmare.