1. If you have a blocked nose, and can't breathe through it, then i would advise waiting till it clears up. Becuase to get the best results with qigong you need to have your tongue touching the roof of your mouth, inhaling and exhaling through the nose.
2. Though qigong does boost the immune system, it takes time to build up proficiency. Qigong is also just one aspect of lifestyle choices. If a person is doing other things that compromise the immune system, such as
Qigong is not magic, you need to be working with your immune system, not against it, in all aspects of life. But also we can all fall prey to infections now and again.
I can't speak for advanced practitioners, but I've been practicing medical qigong for five years, and this fall and winter as others where all coming down with colds and flu i passed through totally unharmed.
Hope this is of some use.....