A systematic review and Meta-analysis confirmed that regular moderate-to vigorous-intensity physical activity not only reduces the risk of community-acquired infectious diseases and infectious disease mortality, but also reduces the risk of infection, it also strengthens the immune system’s first line of defense (e.g., by increasing CD4 count and salivary IGA concentration), effects of Regular Physical Activity on the Immune System, Vaccination and Risk of Community-Acquired Infectious Disease in the General Population: Systematic Review and meta-analysis. Here, CD4 is linked to infectious disease.