Physical activity (PA) modulates immune system functions. Susceptibility to viral infection thereby depends on the volume and intensity of PA: regular moderate PA (up to 60% of maximal oxygen uptake for no more than 1 hour/session) and an associated high cardiovascular fitness (CRF) boost immune system capacity and reduce inflammation, whereas exhausting PA transiently suppresses immune functions. It seems reasonable that the beneficial effects of regular PA on immune function are at least partly mediated by mitochondrial fitness, as outlined further. Both low mitochondrial fitness and low CRF may be important risk factors for the ongoing COVID-19 pandemic, possibly representing a link between established risk factors like age and various chronic diseases, such as obesity and diabetes mellitus.
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