Does extreme exercise weakens immune system?

theglobeandmail.com - New research casts doubt on idea that extreme exercise weakens immune system

See also:
Training-related and competition-related risk factors for respiratory tract and gastrointestinal infections in elite cross-country skiers
Exercise Intensity and Duration Effects on In Vivo Immunity
Recommendations to maintain immune health in athletes
Anxiety and perceived psychological stress play an important role in the immune response after exercise.