Why do I keep getting ill? Recurring infections and what to test

Vitamin D deficiency and immune regulation

Vitamin D is arguably the single most important nutrient for immune function, and deficiency is widespread in the UK due to limited sunlight exposure, particularly between October and April. Vitamin D activates T cells, the white blood cells that identify and destroy infected cells, and stimulates the production of antimicrobial peptides that form part of the innate immune defence. People with vitamin D levels below 25 nmol/L have significantly higher rates of respiratory tract infections compared to those with adequate status. Unlike the acute immune response to a specific pathogen, vitamin D deficiency creates a background vulnerability across all infection types, making it the first marker to test when recurring illness is a pattern.

Iron deficiency and white blood cell production

Ferritin, which reflects stored iron, is directly required for the production and function of immune cells. Iron deficiency impairs the proliferation of lymphocytes, the white blood cells that produce antibodies and coordinate the adaptive immune response. Low ferritin is common in menstruating women, people following plant-based diets, and those with poor dietary intake or impaired absorption. People with ferritin below 30 mcg/L often report persistent fatigue alongside increased susceptibility to infection, reflecting the overlapping effects of iron deficiency on both energy production and immune cell function.

Chronic stress and cortisol-mediated immunosuppression

Prolonged psychological stress elevates cortisol, which is broadly immunosuppressive at sustained levels. Chronic cortisol elevation reduces the production of lymphocytes, inhibits antibody production, and impairs the function of natural killer (NK) cells, the immune cells responsible for destroying virally infected cells. Studies in psychoneuroimmunology have consistently found that people experiencing chronic stress have higher rates of both upper and lower respiratory infections, slower wound healing, and reduced vaccine response compared to those with lower stress burden. The immune consequences of chronic stress are measurable in blood and are a significant but often overlooked contributor to recurring illness.

Blood sugar dysregulation and HbA1c

Elevated blood glucose, even at levels that do not meet the diagnostic threshold for diabetes, impairs white blood cell function in multiple ways. High glucose reduces neutrophil activity (neutrophils are the immune cells that engulf and destroy bacteria), promotes the growth of bacteria and fungi in tissues, and impairs the complement system, which forms part of the immune response against infection. People with HbA1c in the prediabetic range are at meaningfully higher infection risk, and this relationship exists even for those with no awareness of any blood sugar dysregulation.

Gut microbiome and immune priming

Approximately 70% of the immune system is housed in and around the gut. The gut microbiome plays a central role in training and calibrating immune responses, educating white blood cells to distinguish between harmless substances and genuine threats, and maintaining the mucosal barrier that prevents pathogens from crossing into the bloodstream. Disrupted microbiome diversity, sometimes called dysbiosis, impairs immune priming and has been associated with both increased susceptibility to respiratory infections and a slower recovery rate. The gut-lung axis, a bidirectional communication between gut microbiome composition and lung immune responses, is an area of significant current research.

Vitamin B12, folate, and immune cell production

Both B12 and folate are required for the rapid cell division involved in producing white blood cells during an immune response. Deficiency in either nutrient slows this process, reducing the body's ability to mount an effective response to new infections. B12 deficiency can be subclinical, producing below-optimal immune function without classic neurological or haematological symptoms, particularly in people who are borderline deficient rather than severely low. Vegetarians, vegans, and older adults are at the highest risk of B12 insufficiency.

There is no single test for immune function. What useful blood testing can do is identify the specific deficiencies, metabolic abnormalities, and inflammatory patterns most commonly responsible for increased infection susceptibility. For most people with recurring illness, the relevant workup covers vitamin D, ferritin, B12, full blood count (which includes white blood cell count and differential), CRP, and HbA1c.

A full blood count shows the total number of white blood cells and their subtypes (neutrophils, lymphocytes, monocytes, eosinophils, basophils). Counts outside the normal range can signal either an active immune response or a chronic suppression of immune cell production. Alongside this, the nutritional markers that fuel immune cell production and function give a more complete picture than white cell count alone.

In cases where recurring infections are severe, unusual in type (such as repeated fungal infections or infections that would not normally affect healthy adults), or associated with very low white blood cell counts, your GP should evaluate for primary immunodeficiency conditions, which are rare but require specific investigation beyond standard blood panels. Home blood testing is most useful for people investigating the common, correctable causes of immune vulnerability in otherwise healthy adults.

Optimising vitamin D and key nutrients

For most UK adults, vitamin D supplementation during autumn and winter is both well-evidenced and low risk. The NHS recommends 10 mcg (400 IU) daily as a minimum, but many immunologists and longevity researchers suggest higher targets for optimal immune function, particularly for those who are deficient. Testing before and after supplementation is the most reliable way to confirm that your specific dose is achieving adequate blood levels. Alongside vitamin D, maintaining ferritin above 50 mcg/L and ensuring B12 is in the upper half of the reference range supports immune cell production and function at the level your biology needs.

Sleep and immune restoration

Sleep is not a passive state for the immune system: it is the primary period during which cytokines are released, immune memory is consolidated, and T cell activity is at its peak. Consistently getting less than 7 hours of sleep significantly increases the risk of catching an upper respiratory infection when exposed to a virus, an effect that has been demonstrated in controlled exposure studies. Sleep timing consistency, darkness during sleep, and avoiding alcohol (which disrupts sleep architecture even when it promotes initial drowsiness) are the most impactful non-pharmaceutical tools for immune support.

Gut microbiome diversity

Because the gut microbiome trains and calibrates immune responses, supporting microbiome diversity is a direct route to improving immune resilience. Eating 30 or more different plant species per week is the most evidence-based dietary approach to microbiome diversity. Fermented foods including yoghurt, kefir, kimchi, and sauerkraut have been shown to reduce inflammatory markers and increase microbiome diversity in randomised controlled trials. For people with recurring infections, assessing microbiome composition through a gut test and tracking how diversity changes with dietary interventions provides a measurable signal of whether your approach is working.

Blood sugar management

Reducing post-meal blood glucose spikes through dietary approaches, including reducing refined carbohydrates, prioritising protein and fibre at meals, and timing carbohydrate intake around physical activity, directly improves neutrophil function and reduces the tissue environment in which bacteria and fungi proliferate. These changes do not require a diabetes diagnosis to be worth pursuing: even modest improvements in HbA1c within the normal range are associated with improved infection outcomes and recovery speed.