Which factors weaken your immune system?

Nutritional deficiencies are the best-supported cause of a weakened immune system. A vitamin A deficiency damages the mucous membrane barriers and reduces antibody production; in children and pregnant women this demonstrably leads to higher mortality in areas where vitamin A is scarce. A zinc deficiency also increases the risk of serious infections such as diarrhoea and pneumonia, two of the biggest causes of death in children under five worldwide. Iron deficiency, the most common nutritional deficiency in the world, impairs the immune system as well, on top of the well-known anaemia it causes.

Certain medical conditions suppress the immune system severely. Organ or stem-cell transplantation, an HIV infection, and prolonged use of high-dose steroids (more than 20 mg of prednisone per day) more than double to triple the risk of serious pneumonia. The risk of dying from such pneumonia is also considerably higher. These are the most severe forms of immune suppression that are medically recognised.

Psychosocial stress and mental state also play a role. Bereavement following the loss of a loved one and psychiatric conditions have been linked in multiple studies to measurable weakening of the immune system. Whether that weakening actually leads to illness more often has not yet been fully clarified, but psychological factors have repeatedly been associated with an increased susceptibility to respiratory infections, cardiovascular disease and rheumatoid arthritis.

Prolonged strenuous exercise, such as running a marathon, temporarily weakens the immune system. In the three to 72 hours that follow, the so-called 'open window', viruses and bacteria can take hold in the body more easily. Severe cold exposure can also suppress several components of the immune system, but this has been demonstrated primarily in animal experiments; in humans, adaptation occurs after two to three weeks.

Ageing remodels the immune system. The thymus, the gland that produces new immune cells, declines in function. This leads to fewer new, untrained T-cells. Memory cells, however, can continue to function more strongly, so it is not a straightforward decline. Oxidative stress, an excess of harmful oxygen particles in the body, contributes to the wear and tear of immune cells, but the evidence that antioxidants meaningfully counteract this is limited.