Individual differences in immune function, genetics, lifestyle, and past microbial exposure all combine to create a wide spectrum of susceptibility to illness. There is no single reason one person catches every cold that passes through the office while their colleague sails through winter unscathed. The explanation involves at least a dozen overlapping factors, some fixed at birth and others surprisingly changeable, and the science paints a picture that is more interesting than “some people just have better immune systems.”
Genetic Variation Sets the Baseline
Your DNA writes the first draft of your immune defenses. The genes that code for immune-system proteins vary dramatically from person to person, and those differences shape how well your body recognizes and fights off specific pathogens. Genetic diversity across ethnic and racial groups contributes to differences in disease susceptibility, autoimmune disorders, and even cancer risks in specific populations, with environmental factors like geography and socioeconomic conditions further shaping those immune responses.1PubMed Central. Human immune system: Exploring diversity across individuals and populations Some of this variation comes from a cluster of genes involved in presenting pieces of invaders to immune cells. Because these genes are highly variable across individuals, two people exposed to the same virus can mount very different responses. One person’s immune cells might recognize the threat immediately; another’s might be slower to respond.
Biological sex adds another genetic layer. Males tend to be more susceptible to the acute effects of viral disease and certain cancers, while females generally mount a stronger immune response. That heightened response comes with a trade-off: it can tip into autoimmune complications, where the immune system mistakenly attacks the body’s own tissues.2PubMed Central. Sex differences in tissue-specific immunity and immunology The interplay of sex hormones, X-chromosome gene dosage, and environmental factors makes this more complicated than a simple “women have stronger immunity” headline, but the pattern holds broadly across studies. If you have ever noticed that men in your household seem to suffer more dramatically when they catch a respiratory virus, there is some biological basis for that observation beyond stereotypes about “man flu.”
The Gut Microbiome and What Happens in Early Life
Trillions of microorganisms live in your intestines, and they do far more than help you digest food. Symbiotic gut bacteria promote immune balance, enhance immune responses, and protect against pathogen colonization through several mechanisms, including direct killing of harmful microbes, competition for nutrients, and stimulating the immune system to stay alert.3PubMed Central. Gut microbiota: Role in pathogen colonization, immune responses, and inflammatory disease A healthy, diverse gut microbiome essentially functions as a second immune organ. People whose microbiomes have been disrupted, whether by repeated antibiotic use, poor diet, or other factors, lose some of that protection.
When this microbial community takes root matters enormously. Early postnatal life is a critical window for both immune-system development and microbial colonization. Research in animal models has shown that disrupting the microbiome during this period can cause immune defects that persist into adulthood and increase susceptibility to certain diseases.4PubMed Central. Correlation between early-life regulation of the immune system by microbiota and allergy development This isn’t just about infection risk. Early microbial colonization of mucosal tissues plays a central role in educating the immune system, helping it develop tolerance to harmless environmental exposures. When that education is disrupted, the consequences can include inflammatory bowel disease, allergy, and asthma later in life, and researchers have identified a critical developmental window during which these effects can become irreversible.5PubMed Central. How colonization by microbiota in early life shapes the immune system
More specifically, early microbial exposure appears to alter the architecture of certain immune cell populations. Research has shown that microbial contact early in life drives the preferential expansion of highly responsive immune cells that persist into adulthood and provide enhanced protection against intracellular pathogens.6PubMed Central. Early microbial exposure shapes adult immunity by altering CD8+ T cell development Children who grow up in extremely sterile environments, who receive many courses of antibiotics before age two, or who are born by cesarean section and miss the microbial transfer from the birth canal can end up with an immune system that was never properly “trained” during its most receptive phase.
Stress, Sleep, and the Quiet Saboteurs
Chronic psychological stress does not just make you feel lousy. It rewires your immune system’s ability to regulate inflammation. A compelling pair of viral-challenge studies found that people who had recently experienced a prolonged threatening stressor developed resistance in the receptors that normally dial down inflammation. That receptor resistance, in turn, predicted a higher risk of developing a cold after deliberate virus exposure, and among those who did get infected, it predicted greater production of inflammatory molecules in the airways.7PubMed Central. Chronic stress, glucocorticoid receptor resistance, inflammation, and disease risk In other words, stress didn’t just increase the chance of getting sick; it made the inflammatory response to infection worse once it arrived.
Chronic stress can also reactivate latent viruses that most people carry without knowing it. Cytomegalovirus, for example, infects a majority of adults worldwide and normally sits dormant. Under chronic social stress, this virus can reactivate, accelerating immune aging and contributing to chronic disease.8PubMed Central. Chronic Stress and Latent Virus Reactivation: Effects on Immune Aging, Chronic Disease Morbidity, and Mortality The combination of latent viral reactivation and ongoing stress creates a feedback loop that speeds up the decline of immune function over time.
Sleep loss does something similar. Sleep supports host defense against infection and inflammatory insults, and when you consistently shortchange it, the immune system shifts toward a chronic inflammatory state. Sleep deprivation has been linked to changes in both the fast-acting and slower-adapting branches of immunity, raising the risk for infectious and inflammatory diseases across a surprisingly broad range, from cardiometabolic conditions to autoimmune disorders.9PubMed Central. Role of sleep deprivation in immune-related disease risk and outcomes The person who “never gets sick” and the person who catches everything may be sleeping very different amounts, and that gap alone can be a substantial part of the explanation.
Exercise, Body Weight, and Metabolic Health
Regular moderate exercise is associated with fewer infections compared with being sedentary. But the relationship between exercise and immunity has a twist: prolonged bouts of intense exercise temporarily depress several aspects of immune function, including the ability of certain white blood cells to kill pathogens and to present foreign material to the rest of the immune system. This dip typically lasts roughly three to twenty-four hours after the exercise session, depending on its intensity and duration.10PubMed. Immune function in sport and exercise Competitive endurance athletes sometimes experience more upper respiratory infections during heavy training blocks for exactly this reason. Moderate, consistent activity appears to be the sweet spot for immune support.
Body weight matters independently of exercise habits. In people carrying significant excess body fat, the adipose tissue itself becomes a source of chronic, low-grade inflammation. Immune cells accumulate in fat tissue, and the balance between pro-inflammatory and anti-inflammatory cell types shifts toward inflammation, driving a sustained immune response that resembles the body’s reaction to an infection even when no pathogen is present.11JCI Insight. Inflammatory mechanisms linking obesity and metabolic disease This chronic background inflammation doesn’t just raise the risk for metabolic disease; it can also impair the immune system’s ability to respond effectively when a real infection does come along, because resources and signaling pathways are already occupied fighting a phantom threat.
Age and the Gradual Decline of Immune Defenses
Getting older changes the immune system in two related ways. The fast-acting, innate immune system drifts toward a state of persistent low-level inflammation, sometimes called inflammaging. Meanwhile, the slower, more targeted adaptive immune system gradually loses its ability to mount strong, specific responses to new threats. These two processes feed each other, leaving older adults more vulnerable to viral, bacterial, and opportunistic infections, and also less protected after vaccination.12PubMed Central. The Impact of Immune System Aging on Infectious Diseases
This helps explain why an illness that a twenty-year-old shrugs off in a few days can put a seventy-year-old in the hospital. The older immune system is not just slower; it is working with a depleted repertoire of cells capable of recognizing novel pathogens. It is also dealing with the cumulative effects of decades of stress, latent virus reactivation, and metabolic wear, all of which compound the age-related decline.
Trained Immunity and the Benefits of Past Exposure
The adaptive immune system’s ability to “remember” past infections through antibodies and memory cells is well known, but researchers have discovered that the innate immune system has its own form of memory. After encountering certain microbes, innate immune cells undergo lasting changes in how their genes are read and how their metabolism functions, making them more responsive to future challenges from a wide variety of pathogens, not just the original one.13PubMed. Trained immunity: adaptation within innate immune mechanisms This phenomenon, called trained immunity, means that your history of infections and vaccinations does more than protect you against specific diseases. It can broadly calibrate how ready your frontline immune defenses are.14PubMed Central. Trained immunity: A “new” weapon in the fight against infectious diseases
This has practical implications. Someone who was exposed to a wider range of microbes in childhood may carry a more broadly “trained” innate immune system as an adult. It also means that certain vaccines can have non-specific protective effects beyond the disease they were designed to prevent, a finding that has been observed with the BCG tuberculosis vaccine in several populations. People differ in how much trained immunity they have accumulated, and that difference contributes to the gap between those who rarely get sick and those who seem vulnerable to everything.
Mucosal Barriers and Preexisting Conditions
Before a pathogen ever reaches your bloodstream, it has to get past the mucosal barriers lining your nose, throat, and lungs. These surfaces produce antibodies, particularly a type called IgA, that neutralize microbes on contact. But people differ in how effectively their mucosal surfaces produce these antibodies. Those with selective IgA deficiency or certain antibody subclass deficiencies show measurably lower resistance to respiratory infections.15PubMed. Humoral immune response patterns of human mucosae: induction and relation to bacterial respiratory tract infections Adding insult to injury, several common respiratory bacteria produce enzymes that specifically degrade the most prevalent form of IgA found in the nose and airways, exploiting a vulnerability in the body’s first line of defense.
Chronic conditions like asthma compound the problem. People with asthma tend to have a delayed and weakened antiviral response, likely due to reduced production of a key class of signaling proteins that normally help cells resist viral replication. This impaired response allows viruses to replicate more freely and leaves the underlying allergic inflammation unchecked.16PubMed Central. Asthma and viral infections An intricate relationship The result is that people with asthma often experience respiratory infections more frequently and more severely than those without it, and each infection can in turn worsen their asthma, creating a vicious cycle.
Nutritional Gaps You Might Not Notice
Micronutrient deficiencies can quietly undermine immune function without causing obvious symptoms. A case-control study in children found that 80% of those with urinary tract infections were deficient in vitamin D, compared with about 18% in the healthy control group. Zinc deficiency followed a similar pattern: 60% of the infection group versus about 18% of healthy controls.17PubMed Central. Serum Vitamin D and Zinc Levels in Children with Urinary Tract Infection without Confounding Factors: A Case-Control Study This was a single study in children, so the exact numbers shouldn’t be taken as universal, but the general finding fits a broader pattern in the literature: people who are low in key micronutrients get sick more often, and they often don’t realize they are deficient until a blood test reveals it.
Vitamin D and zinc are far from the only nutrients that matter. Iron, selenium, vitamin C, and vitamin A all play roles in immune cell function. The practical takeaway is that the person who “eats fine” but subsists mostly on processed food can have subtle deficiencies that leave their immune system running at less than full capacity, even when their calorie intake is more than adequate.
Not Everyone Gets Sick the Same Way
Part of the illusion that some people never get sick may be that they get infected without feeling particularly ill. During the COVID-19 pandemic, researchers closely tracked virus shedding in asymptomatic, presymptomatic, and mildly symptomatic patients. Asymptomatic patients shed virus for a median of about 28 days, and mildly symptomatic patients for about 31 days.18PubMed Central. Virus shedding dynamics in asymptomatic and mildly symptomatic patients infected with SARS-CoV-2 These asymptomatic carriers were genuinely infected but experienced no noticeable illness. Similar dynamics play out with common respiratory viruses every year: you can be infected, harbor and spread a virus, and never feel a thing.
What determines whether an infection produces miserable symptoms or flies under the radar? The answer circles back to all of the factors covered above, including genetics, trained immunity, nutritional status, and the balance of the immune response. Someone whose immune system clears a virus efficiently and without excessive inflammation may not even register the encounter as “being sick.” Their coworker, facing the same virus with a slightly different immune profile, might spend a week on the couch. Both were infected. Only one experienced illness.
Why Vaccine Responses Vary Between People
If all of these factors affect how you respond to natural infection, they also affect how you respond to vaccines. Roughly 2 to 10% of healthy people fail to mount protective antibody levels after routine vaccinations. Research into these non-responders found that the failure tends to be specific to particular vaccines rather than reflecting a globally weak immune system, and that certain regulatory immune cells and signaling molecules are involved in dampening the response.19PubMed Central. Primary vaccine failure to routine vaccines: Why and what to do? Non-responsiveness also increases with age, particularly for vaccines against novel pathogens. This is one reason why older adults sometimes get sicker even when fully vaccinated, and why some vaccine schedules call for higher doses or additional boosters for people over 65.
Evolutionary Trade-offs in Immune Strength
It might seem like the ideal immune system would be the most aggressive one, but evolution has not pushed us in that direction, and for good reason. The optimal immune response is the one that best balances the cost of fighting an infection against the cost of the immune response itself. A response that is too weak allows a pathogen to multiply unchecked. A response that is too strong damages the body’s own tissues, a phenomenon visible in severe COVID-19 cases, autoimmune disease, and allergic reactions.20Trends in Immunology. Evolutionary perspectives on immunopathology and the vital balance between protection and pathology
This means that the variation we see in immune responses across the population is not a defect. It is partly the result of evolutionary pressures that selected for a range of immune strategies. Human susceptibility to overzealous, tissue-damaging inflammatory responses has roots in the legacies of multicellularity, our life-history strategy (living long lives with relatively few offspring), and our deep coevolution with the microbes that colonize us.21Trends in Immunology. Evolutionary perspectives on immunopathology and cytokine responses The person who gets every cold but never develops an autoimmune disease and the person who rarely catches anything but develops rheumatoid arthritis at forty may simply sit at different points along a spectrum that evolution has kept deliberately broad.
Exposure Frequency and the Paradox of Contact
Common sense says that people who are exposed to more germs get sick more often. That is generally true, but mathematical modeling work on common cold dynamics adds a wrinkle. Under certain conditions related to the characteristics of a virus and the population it circulates in, increased exposure can actually result in a lower average disease burden across a community. Both population-level and individual-level modeling approaches have produced this counterintuitive result.22PubMed Central. Impact of exposure frequency on disease burden of the common cold – A mathematical modeling perspective The mechanism involves the buildup of specific immunity in the population: when people encounter a pathogen more frequently, a larger share develops protective immunity, which eventually reduces overall circulation. This is, in a sense, a population-level version of trained immunity, and it helps explain why daycare workers and parents of young children sometimes seem to power through cold season better after a rough first year or two of constant exposure.