Inclusion Criteria:  identifies as exercising 1-3 hours per week or more than 6 hours per week, on average (last 6 months) meets American College of Sports Medicine criteria for participation in exercise  Exclusion Criteria:  underlying medical problems that contraindicate supervised high intensity exercise past or present history of autoimmune disease, HIV, hepatitis, stroke, or cardiovascular disease chronic/debilitating arthritis central or peripheral nervous disorders asthma, emphysema, or bronchitis bedridden in the past three months history of blood clotting disorders administration of any medication that might affect physiological response to exercise functional or cognitive impairment that would limit exercise performance or prohibit informed consent having any common-cold or influenza like symptoms (scratchy throat, runny nose, inflamed sinuses, frequent sneezing or coughing) in the last 2 weeks pregnancy tobacco use