Abstract：At present, the crisis of the global pandemic of coronavirus disease 2019 (COVID-19) is mitigating, with symptoms tending to become as mild as influenza. As it continues to mutate, the adverse impact on human health may remain for a period of time. Increased physical activity and regular exercise enhances cardiorespiratory fitness, which in turn lowers risks of cancer, cardiovascular disease, and all-cause mortality. Improved cardiorespiratory fitness also lowers the severity and mortality of communicable diseases such as upper respiratory tract infection and pneumonia. The purpose of this paper is to explore published research regarding the impact of both good and poor cardiorespiratory fitness on the risk of severity and mortality after infection with COVID-19, and any potential mechanisms. Data was collected via the Google Scholar search engine using keywords cardiorespiratory fitness, exercise, COVID-19, and mortality. Results: evidence derived from epidemiological research confirm that increasing physical activity and regular exercise and improving cardiorespiratory fitness lowers the risks of hospitalization, admission to the intensive care unit, and mortality after being infected with COVID-19. Improved cardiorespiratory fitness through increased physical activity and regular exercise likely promotes immune surveillance and defense; reduces the pathway of viral entry into cells; improves anti-inflammatory and anti-oxidant capacity; increases mitochondrial biogenesis, density, number, structure, and function; and subsequently reduces cytokine storm and protects the human body from severe damage by COVID-19. (Full text)