The immune system relies on a network of cells, tissues and organs to defend the body against viruses, bacteria and other threats. Its effectiveness can be influenced by factors including sleep, nutrition, physical activity, stress and alcohol use.
Some commonly cited “bad habits,” however, do not have the same evidence behind them. Nail biting, for example, can damage the skin around the nails and increase the risk of local infection, but it should not be described as a direct cause of a weakened immune system.
Research on body-focused repetitive behaviors, which include nail biting and hair pulling, also does not support the claim that about 30% of people engage in both behaviors. A 2023 study of 1,481 people found that 11.4% met the study’s criteria for nail-biting disorder, while prevalence estimates vary substantially between studies and populations.
How the immune system works
The immune system is the body’s defense network. It includes white blood cells, tissues and organs that recognize and respond to harmful organisms and other foreign substances.
White blood cells are among the cells involved in identifying and responding to infections. Other parts of the immune system include physical barriers such as the skin and the cells lining the digestive tract.
Prolonged stress
Stress is a normal response that helps the body deal with immediate challenges. Problems can arise when stress is prolonged.
The body’s stress response involves hormones including cortisol. Research from the National Institutes of Health has found that prolonged activation of the stress system can affect immune activity and may reduce the body’s ability to respond effectively to infection.
That does not mean that experiencing stress will automatically make someone sick. The relationship between stress and immunity is more complex and depends on factors including the duration and severity of the stress.
Poor sleep
Sleep also plays a role in immune function. The National Heart, Lung, and Blood Institute says different parts of the immune system become active during sleep, and people who do not get enough sleep may be more likely to develop colds and other infections.
An NIH-funded study also found that consistent sleep supports processes involved in the production and development of certain immune cells.
Poor nutrition
A balanced diet provides the nutrients the immune system needs to function normally.
The National Institutes of Health says adequate amounts of vitamins and minerals, including vitamins A, B6, B12, C, D and E, folate, iron, selenium and zinc, are important for immune function. Deficiencies in some of these nutrients can impair immune responses and increase susceptibility to infections.
This does not mean that taking extra vitamins automatically prevents illness. For people who are not deficient, routine supplementation generally has limited evidence for preventing or treating specific infections.
Heavy alcohol use
Heavy alcohol use can interfere with the body’s immune response.
The National Institute on Alcohol Abuse and Alcoholism says both acute and chronic heavy alcohol use can affect several parts of the immune system, impairing the body’s defense against infection and recovery from injury.
Physical activity
Regular physical activity is another part of maintaining overall health and supporting immune function.
The Centers for Disease Control and Prevention says emerging research suggests physical activity may help support immune function. Regular activity also has established benefits for sleep, anxiety, cardiovascular health and the prevention of several chronic diseases.
The CDC recommends that adults generally aim for at least 150 minutes of moderate-intensity physical activity or 75 minutes of vigorous activity each week, although recommendations can vary according to age and health status.
Supporting immune health
There is no single habit that guarantees protection from infection. Instead, immune health is supported by a combination of behaviors.
Eating a varied diet, getting enough sleep and staying physically active can help the immune system function normally. Avoiding smoking and excessive alcohol use also supports immune health. Vaccination remains important because healthy habits alone do not train the immune system to recognize specific diseases.
The evidence therefore points away from a simple list of “bad habits” that weaken immunity. Immune function is influenced by multiple factors, and maintaining healthy routines is one part of supporting the body’s natural defenses.
