This review will provide an overview of the immune system and then describe the effects of
frailty,
obesity, specific
micronutrients and the gut microbiota on immunity and susceptibility to
infection including data from the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic where relevant. A key role for the immune system is providing host defence against pathogens. Impaired immunity predisposes to
infections and to more severe
infections and weakens the response to vaccination. A range of nutrients, including many
micronutrients, play important roles in supporting the immune system to function. The immune system can decline in later life and this is exaggerated by
frailty. The immune system is also weakened with
obesity, generalised
undernutrition and
micronutrient deficiencies, which all result in increased susceptibility to
infection. Findings obtained during the SARS-CoV-2 pandemic support what was already known about the effects of ageing,
frailty and
obesity on immunity and susceptibility to
infection. Observational studies conducted during the pandemic also support previous findings that multiple
micronutrients including
vitamins C, D and E,
zinc and
selenium and long-chain
n-3 fatty acids are important for immune health, but whether these nutrients can be used to treat those already with coronavirus disease discovered in 2019 (COVID-19), particularly if already hospitalised, is uncertain from current inconsistent or scant evidence. There is gut
dysbiosis in patients with
COVID-19 and studies with probiotics report clinical improvements in such patients. There is an inverse association between adherence to a healthy diet and risk of
SARS-CoV-2 infection and hospitalisation with
COVID-19 which is consistent with the effects of individual nutrients and other dietary components. Addressing
frailty,
obesity and
micronutrient insufficiency will be important to reduce the burden of future pandemics and nutritional considerations need to be a central part of the approach to preventing
infections, optimising
vaccine responses and promoting recovery from
infection.