Dose-response relationship between weekly physical activity level and the frequency of colds in Chinese middle-aged and elderly individuals 1The School of Sports Medicine and Rehabilitation, Beijing Sports University, Beijing, China 2Laboratory of Sports Stress and Adaptation of General Administration of Sport, Beijing Sport University, Beijing, China 3The School of Sports Coaching, Beijing Sports University, Beijing, China DOI 10.7717/peerj.17459 Academic Editor Luca Correale Subject Areas...