Frequent infections usually point to one or more factors weakening your immune defenses, whether that’s an underlying medical condition, a medication side effect, chronic stress, a nutritional gap, or simply the natural changes that come with aging. Most people get a couple of colds or minor infections each year without concern. But if you’re cycling through antibiotics more than twice a year, developing pneumonia more than once, or getting three or more sinus infections annually, something beyond bad luck is likely at play.
How Many Infections Are Too Many
Not every winter cold means your immune system is failing. The American Academy of Allergy, Asthma & Immunology uses specific thresholds to flag a possible immune deficiency: more than two courses of antibiotics per year in adults, more than three episodes of bacterial sinusitis in a single year, or pneumonia twice over any time period. Chronic sinusitis that never fully clears also raises concern. These benchmarks matter because they separate the “I catch everything going around the office” feeling from a pattern that warrants medical investigation.
Medical Conditions That Weaken Immunity
A long list of chronic diseases can quietly erode your immune function. Diabetes is one of the most common culprits. Elevated blood sugar impairs the ability of white blood cells to reach and destroy bacteria, which is why people with poorly controlled diabetes are more susceptible to skin infections, urinary tract infections, and pneumonia.
Chronic kidney disease and liver disease both compromise immunity through different mechanisms. Kidney disease leads to a buildup of waste products that blunt immune cell activity, while liver disease reduces the production of proteins your immune system depends on. Autoimmune conditions like lupus can paradoxically leave you more vulnerable to infections, partly because the immune system is misdirected and partly because the treatments suppress it further. Blood cancers such as leukemia and lymphoma directly disrupt the production of healthy immune cells.
Certain viral infections also take a toll. HIV is the most well-known, but Epstein-Barr virus, cytomegalovirus, and even a severe case of measles can temporarily or permanently reset aspects of immune memory, leaving you less protected against infections you’d normally fight off easily.
Genetic Immune Disorders Diagnosed in Adulthood
Some people are born with immune systems that don’t work quite right, but the problem doesn’t become obvious until their 20s, 30s, or even 40s. The most common of these is common variable immunodeficiency (CVID), a condition where the body fails to produce enough protective antibodies. Most patients are diagnosed between ages 20 and 45, often after years of bouncing between doctors and accumulating hospitalizations.
Over 90% of people with CVID experience recurrent respiratory infections: pneumonia, sinusitis, and ear infections, typically caused by common bacteria. But the condition isn’t limited to infections. Some people develop autoimmune problems, chronic diarrhea, enlarged lymph nodes, or a type of inflammatory lung disease before anyone thinks to check their antibody levels. If you have recurrent respiratory infections alongside autoimmune symptoms or unexplained digestive issues, CVID is worth considering. Milder genetic variants of more severe immune disorders can also fly under the radar into adulthood, presenting with a toned-down version of symptoms that doctors may not immediately recognize.
Medications That Suppress Your Defenses
If you’re taking certain medications, your infection pattern may be a predictable side effect rather than a mystery. Corticosteroids like prednisone are among the most widely prescribed drugs that suppress immune function. Even short courses dampen the activity of multiple types of immune cells, and long-term use significantly raises infection risk.
Immunosuppressants used after organ transplants or for autoimmune diseases, including cyclosporine, tacrolimus, and azathioprine, all carry infection as a major side effect. Biologic therapies like rituximab, used for conditions ranging from rheumatoid arthritis to certain cancers, can reactivate dormant viruses like hepatitis B. Chemotherapy drugs broadly suppress bone marrow function, reducing the white blood cells you need to fight off pathogens. If your infections started or worsened after beginning a new medication, that connection is worth raising with your prescriber.
How Chronic Stress Suppresses Immunity
Stress isn’t just a feeling. Prolonged psychological stress triggers sustained production of cortisol, your body’s primary stress hormone, and cortisol at chronically elevated levels actively dismantles immune function. It reduces the number of active immune cells circulating in your blood, suppresses the production of signaling molecules that coordinate immune responses, and decreases the activity of natural killer cells, which are your first line of defense against virus-infected cells.
Cortisol also disrupts the communication between different branches of your immune system, so even the cells that remain active can’t coordinate effectively. This isn’t a subtle effect. People under chronic stress, whether from caregiving, financial hardship, grief, or demanding work, consistently show higher rates of respiratory infections and slower wound healing. The immune suppression from ongoing stress is comparable in some ways to taking a mild immunosuppressive drug.
Nutritional Gaps and Sleep
Your immune system requires specific raw materials to function, and running low on any of them creates vulnerabilities. Vitamin D is one of the most studied. Serum levels below 20 ng/mL are considered deficient, and levels between 20 and 30 ng/mL are insufficient. Both ranges are associated with impaired immune function and greater susceptibility to respiratory infections, including severe pneumonia. Given that an estimated 40% or more of adults in northern climates have insufficient vitamin D levels, this is a remarkably common and correctable contributor to frequent infections.
Zinc, iron, and vitamin C all play distinct roles in immune cell production and function. Alcohol use disorder and general undernutrition are recognized causes of secondary immunodeficiency. Even moderate but persistent calorie restriction or protein deficiency can impair antibody production. Sleep deprivation compounds the problem: consistently getting fewer than six hours reduces the production of protective proteins and lowers the activity of immune cells that target specific pathogens.
Aging and Immune Decline
The immune system undergoes predictable changes with age, a process researchers call immunosenescence. The thymus, a small organ behind your breastbone where immune cells mature, begins shrinking around puberty and is gradually replaced by fat tissue. By age 50, your production of new immune cells is less than 10% of its peak levels. This means you have fewer “naïve” cells capable of recognizing new threats and a growing proportion of memory cells trained against infections you’ve already encountered.
The practical result is a reduced ability to fight off unfamiliar pathogens, weaker responses to vaccines, and a baseline state of low-grade inflammation that paradoxically coexists with diminished immune effectiveness. Cells that normally engulf and destroy bacteria, like neutrophils and macrophages, become sluggish with age, showing reduced ability to migrate to infection sites and kill pathogens. This is why older adults are disproportionately affected by pneumonia, influenza, and urinary tract infections. These changes are normal, but they can be accelerated by the other factors on this list.
Environmental Exposures
Sometimes the issue isn’t your immune system at all, but what it’s being exposed to. Spending time in damp or water-damaged buildings exposes you to mold spores that can cause symptoms mimicking recurrent infections. The CDC notes that people in mold-affected environments report respiratory symptoms, cough, muscle aches, fever, chills, and extreme fatigue. One condition, hypersensitivity pneumonitis, looks and feels like pneumonia but doesn’t respond to antibiotics, which can lead to repeated courses of treatment for what appears to be a stubborn lung infection.
If your infections cluster around time spent in a particular building, or if antibiotics never seem to fully resolve your symptoms, environmental exposure is worth investigating. Poor indoor air quality, occupational chemical exposures, and even heavy secondhand smoke exposure can all irritate airways and compromise local immune defenses in the lungs and sinuses.
What Testing Looks Like
If your infection pattern crosses the thresholds mentioned earlier, a doctor can run targeted blood work to assess your immune function. The process typically starts simple: a complete blood count with differential, which reveals whether you have enough white blood cells and whether the proportions of different cell types look normal. Severe drops in lymphocytes (a specific type of white blood cell) are a red flag that prompts deeper investigation.
The next step usually involves measuring your immunoglobulin levels, the antibodies (IgG, IgA, IgM) that neutralize bacteria and viruses. If those levels are low or borderline, doctors can test how well you respond to vaccines. This involves giving you a standard vaccination, like a tetanus or pneumonia shot, and checking your blood three to four weeks later to see if your body mounted an appropriate antibody response. A poor response despite normal-ish antibody levels is a hallmark of subtle immune deficiency. HIV testing is standard in any workup for unexplained immune weakness, typically using a viral load test rather than an antibody test for greater accuracy. Additional testing for complement system defects or neutrophil function may follow depending on the types of infections you’re experiencing.