Your risk of infection depends on a combination of factors, some within your control and some not. Age, chronic disease, medications, stress levels, sleep, nutrition, alcohol use, and the health of your gut bacteria all play measurable roles in how well your immune system detects and fights off pathogens. Understanding these factors can help you recognize where your own vulnerabilities lie.
Age and Immune Decline
The immune system weakens gradually with age through a process called immunosenescence. The thymus, a small organ behind the breastbone that trains immune cells, shrinks over time. This means the body produces fewer “naïve” immune cells, the ones capable of recognizing new threats. Instead, the immune system becomes increasingly populated by older, worn-out cells that have encountered so many infections over the years that they’ve lost their flexibility.
Several things go wrong at the cellular level. The diversity of receptors on your immune cells narrows, making it harder to identify unfamiliar pathogens. Stem cells in the bone marrow shift toward producing cells involved in inflammation rather than cells that fight specific infections. Aging immune cells also develop a pattern of releasing inflammatory signals constantly, creating a background hum of inflammation that, paradoxically, makes the immune system less effective when a real threat appears. Damaged energy-producing structures inside cells and a buildup of harmful molecules called reactive oxygen species accelerate the decline further.
Chronic Diseases, Especially Diabetes
Diabetes is one of the strongest chronic-disease risk factors for infection. Elevated blood sugar impairs the ability of white blood cells to reach and destroy bacteria, and it creates a more hospitable environment for pathogens to grow. A large matched cohort study of people with type 1 diabetes found that infection-related hospitalizations increased dramatically as long-term blood sugar control worsened. Those with the poorest control (HbA1c above 97 mmol/mol) had roughly four times the rate of infection-related hospitalizations compared to those with well-managed levels (53 mmol/mol or below). Greater fluctuation in blood sugar over time independently raised the risk as well.
Other chronic conditions, including kidney disease, liver disease, heart failure, and lung disease, also compromise immune defenses, often through a combination of poor circulation, chronic inflammation, and the medications used to manage them.
Medications That Suppress Immunity
Corticosteroids, commonly prescribed for autoimmune conditions, asthma flares, and severe inflammation, are among the most widely used drugs that dampen the immune response. They work by broadly dialing down immune activity, which helps control the disease being treated but also leaves the door open for opportunistic infections.
A large international registry study of hospitalized COVID-19 patients illustrates the tradeoff. After adjusting for other health factors, patients who received corticosteroids early in their hospital stay had 15% higher odds of developing a secondary infection compared to those who did not. The risk was especially pronounced for bloodstream infections (43% higher adjusted odds) and infections from an unknown source (63% higher adjusted odds). Chemotherapy, organ transplant drugs, and biologic therapies for autoimmune diseases carry similar or greater immunosuppressive effects.
Poor Sleep
Sleep is not a passive rest period for your immune system. During sleep, the body ramps up production of signaling molecules that coordinate immune defenses and consolidate immunological “memory” of past threats. Cutting sleep short disrupts this process in ways that show up quickly.
Animal research demonstrates just how dangerous severe sleep loss can be. In one study, mice subjected to total sleep deprivation for four days developed a massive, uncontrolled inflammatory response resembling what happens during sepsis, and roughly 80% of them died. The mechanism involved a compound normally produced in the brain during wakefulness (prostaglandin D2) spilling into the bloodstream through a specific transporter in the blood-brain barrier, triggering a flood of immune cells and inflammatory signals throughout the body. While human sleep loss rarely reaches this extreme, even partial sleep restriction over several nights measurably reduces your ability to fight off common viruses like colds and flu.
Chronic Stress
Short bursts of stress can temporarily boost certain immune functions, a remnant of the fight-or-flight response. Chronic stress does the opposite. When cortisol, the body’s primary stress hormone, stays elevated for weeks or months, it directly reduces the proliferation and activity of T cells, the immune cells responsible for identifying and killing infected cells.
Research on chronically stressed patients has confirmed that sustained high cortisol levels are linked to decreased T cell activation and reduced B cell activity, weakening both arms of the adaptive immune response. This means the body becomes slower to recognize infections, less efficient at clearing them, and less responsive to vaccines. The effect is significant enough that chronic psychological stress is now recognized as an independent risk factor for respiratory infections and slower wound healing.
Vitamin D Deficiency
Vitamin D plays a direct role in activating immune cells that serve as the body’s first line of defense against respiratory pathogens. When blood levels of 25-hydroxyvitamin D fall below 20 ng/mL (50 nmol/L), a threshold most research uses to define deficiency, susceptibility to respiratory infections rises noticeably. A systematic review of studies in children found that those who were vitamin D deficient had consistently higher rates of acute respiratory infections, including pneumonia and bronchiolitis. Some studies used a stricter cutoff of 15 ng/mL and found even greater vulnerability at those lower levels.
Deficiency is common in people who get limited sun exposure, have darker skin, are obese, or live at higher latitudes during winter months. Unlike many nutritional factors where the evidence is mixed, the connection between low vitamin D and respiratory infection risk is well established across multiple age groups.
Alcohol Use
Alcohol affects immune function differently depending on the pattern of drinking, but both binge drinking and chronic heavy use increase infection risk through distinct mechanisms.
Binge drinking, defined in research as roughly five to seven drinks within a two-hour window, triggers a wave of immune cell death. T cells undergo programmed self-destruction at elevated rates after an acute alcohol exposure, temporarily thinning out the very cells needed to fight viral and bacterial infections.
Chronic alcohol consumption causes deeper, longer-lasting damage. Animal studies show that sustained drinking impairs the ability of immune cells to migrate from the bloodstream to the site of an infection, essentially slowing the immune system’s response time. In mouse models of influenza, animals that consumed alcohol for four to eight weeks mounted a significantly weaker virus-specific immune response. In tuberculosis models, chronic alcohol consumption led to substantially higher bacterial loads in the lungs. In humans, heavy drinking is associated with higher rates of pneumonia, tuberculosis, HIV progression, and hepatitis C infection.
Disrupted Gut Bacteria
The trillions of bacteria living in your gut do more than digest food. They form a defensive barrier against dangerous microbes through a mechanism called colonization resistance. A diverse community of gut bacteria protects you by consuming the nutrients that invading pathogens need to establish themselves. This is a community-level trait, meaning no single bacterial species provides the protection on its own. It depends on having a wide variety of species working together.
Antibiotics are the most common disruptor of this balance. While they kill the bacteria causing an infection, they also wipe out beneficial species, reducing diversity and creating openings for dangerous organisms. Research has shown that antibiotic treatment significantly increases colonization risk with harmful bacteria like Klebsiella pneumoniae, which can cause severe bloodstream and urinary tract infections. This is why antibiotic-associated diarrhea and Clostridioides difficile infections are so common after courses of broad-spectrum antibiotics. Diets low in fiber, chronic gastrointestinal diseases, and repeated antibiotic courses can all reduce microbial diversity over time, gradually weakening this natural defense.
Obesity
Excess body fat, particularly around the abdomen, creates a state of chronic low-grade inflammation. Fat tissue is not inert storage; it actively releases inflammatory signals that keep the immune system in a constant state of mild activation. Over time, this persistent background inflammation exhausts immune resources and reduces the system’s ability to respond sharply to a new infection. Obesity also impairs vaccine effectiveness and is an independent risk factor for severe outcomes from respiratory infections, surgical site infections, and sepsis.