There are many people in your situation that never gets a cold. Also there are many that has colds constantly as I understand it. The inability to get a cold could be a sign of excellent immunesystem or it could be a form of error in the immunesystem. As you know there are problems with the immunesystem if you have an illness like for example Me/cfs so it could also be a sign that the body is fighting an infection with the help of vitamin C instead of not doing anything about it. Just a thought though.