Which statement correctly summarizes how regular moderate physical activity affects the immune system?

• A. Physical activity affects only cardiovascular fitness and has no direct or indirect effects on immune surveillance, inflammation, or infection outcomes.
• B. Any increase in physical activity beyond complete sedentary behavior uniformly reduces immune function and increases susceptibility to all infections regardless of intensity or frequency.
• C. Regular moderate physical activity enhances immune surveillance, can decrease susceptibility to some infections, and is associated with lower severity of illness compared to severe physical inactivity.
• D. Only high-intensity daily training boosts immune defense meaningfully; moderate activity has no measurable effect on infection susceptibility or severity.

Answer: (C)