Susceptibility describes how likely a person, organism, population, or even a material is to be affected by a particular threat, whether that threat is a virus, a toxin, a psychological stressor, or a phishing email. The concept shows up across nearly every scientific field, and the consistent finding is that susceptibility is rarely fixed. It shifts depending on your genes, your age, what you were exposed to early in life, the microbial community living in your gut, and dozens of other factors that interact in ways researchers are still untangling. Understanding why one person catches a severe infection while another shrugs it off, or why one community suffers disproportionately during a heat wave, comes down to mapping these layers of vulnerability.
How Genetic Variation Shapes Susceptibility to Infection
One of the clearest examples of genetic susceptibility involves the receptors that viruses use to enter human cells. During the COVID-19 pandemic, researchers investigated whether natural variation in the ACE2 gene, which encodes the receptor SARS-CoV-2 latches onto, could explain why some people developed severe disease while others barely noticed an infection. A study in the Journal of Virology identified a specific variant called ACE2 D355N that restricts the interaction between the virus’s spike protein and the receptor, limiting infection both in lab experiments and in animal models.1PubMed Central. Susceptibilities of Human ACE2 Genetic Variants in Coronavirus Infection In other words, people carrying that variant had a built-in molecular shield that made them harder for the virus to infect.
This pattern is not unique to coronaviruses. In bats, which serve as natural reservoirs for many viruses, researchers found that a single amino acid change in a receptor called NPC1 greatly reduces the ability of Ebola virus to bind and enter cells. The study also found signatures of evolutionary pressure concentrated right at the virus-receptor interface, suggesting that some bat populations have evolved reduced filovirus susceptibility over time.2PubMed Central. Filovirus receptor NPC1 contributes to species-specific patterns of ebolavirus susceptibility in bats The takeaway is that genetic variation in receptors can act like a lock-and-key mismatch, making infection harder or easier depending on tiny molecular differences.
The Evolutionary Trade-Off Problem
If certain genetic variants protect against infection, you might wonder why everyone doesn’t carry them. The answer often involves trade-offs. Mutations that helped our ancestors adapt to new environments or diets sometimes come with a cost: increased susceptibility to other diseases. Research published in Cell Reports found that mutations favored by natural selection in the human genome are enriched at sites linked to disease risk in genome-wide association studies, and these trade-offs are often population-specific.3PubMed. Uncovering the extensive trade-off between adaptive evolution and disease susceptibility A variant that helped one ancestral group survive a particular environmental challenge may increase disease risk in their descendants living under different conditions. Evolution, in short, does not optimize for disease-free living. It optimizes for survival and reproduction, and the genetic baggage from those adaptations still shapes who gets sick and who doesn’t.
Early Life Sets the Stage
Your genes set a baseline, but what happens in the womb and during the first years of life can recalibrate your susceptibility to chronic disease for decades. This works through epigenetics, the chemical modifications that sit on top of your DNA and influence which genes are active without changing the genetic code itself. A growing body of evidence shows that environmental exposures during sensitive early-life periods, including poor nutrition, psychological stress, pollution, and substance use, trigger epigenetic changes that program the body toward chronic conditions like cardiovascular disease and type 2 diabetes.4PubMed Central. Epidemiologic evidence for association between adverse environmental exposures in early life and epigenetic variation: a potential link to disease susceptibility
These modifications act as a kind of biological memory. Research in Biochimie described how early-life nutrition and stress alter DNA methylation and histone modifications, creating stable changes in gene activity that persist across the lifespan and can even be passed to the next generation.5PubMed. Early environmental factors, alteration of epigenetic marks and metabolic disease susceptibility A child born during famine, for instance, may carry epigenetic marks that prime the body to hoard calories, an adaptation that becomes a liability in a world of abundant food. The practical implication is that susceptibility to adult diseases is partially written during a window that closes before most people are even aware of it.
Why Aging Amplifies Vulnerability
At the other end of the lifespan, aging steadily erodes the body’s defenses. The immune system undergoes a gradual decline known as immunosenescence, which weakens both the rapid first-response arm and the slower, more targeted adaptive arm. Phagocytic cells become less effective at engulfing invaders, and the production and function of T- and B-cells diminishes.6PubMed Central. Immunosenescence: How Aging Increases Susceptibility to Bacterial Infections and Virulence Factors The result is a compounding vulnerability to infections that younger immune systems handle without much trouble.
Respiratory infections are a prime example. The aging lung accumulates cellular damage and senescent cells, increasing its vulnerability to pathogens. During the COVID-19 pandemic, the sharply higher mortality and hospitalization rates among people over 65 made this age-dependent susceptibility impossible to ignore.7PubMed Central. Respiratory Infections in the Aging Lung: Implications for Diagnosis, Therapy, and Prevention But the relationship between aging and infection risk is not purely immunological. Frailty, chronic conditions, and increased exposure in care settings all contribute, creating a web of factors that makes older adults disproportionately susceptible to infections that circulate in the general population.8PubMed. Ageing and respiratory infections: the airway of ageing
Your Gut Microbiome as Gatekeeper
One of the more surprising determinants of susceptibility lives in your intestines. The community of bacteria in your gut provides what microbiologists call colonization resistance, a set of mechanisms that make it harder for incoming pathogens to establish themselves. These mechanisms include competition for nutrients, secretion of antimicrobial compounds, support for the gut’s physical barrier, and even deployment of bacteriophages that kill invading bacteria.9PubMed Central. Gut Microbiota and Colonization Resistance against Bacterial Enteric Infection When this microbial community is disrupted, by antibiotics, acid-suppressing medications, or other drugs, the door swings open for pathogens to move in.10PubMed Central. Gut Microbiota and Colonization Resistance against Bacterial Enteric Infection
The variation between individuals is striking. In experiments with cholera, researchers found that mice receiving gut bacteria from one human donor were resistant to Vibrio cholerae, while mice receiving bacteria from a different donor were highly susceptible. Crucially, mixing the resistant and susceptible bacterial communities together restored protection, driving pathogen levels below even those seen in the resistant-only group.11Cell. Interpersonal Gut Microbiome Variation Drives Susceptibility to Vibrio cholerae Infection This suggests that susceptibility to gut infections is not a fixed trait but something that could, at least in principle, be modified by changing the microbial landscape.
Mental Health and the Diathesis-Stress Model
Susceptibility plays a central role in how researchers think about depression and other mental health conditions. The diathesis-stress model proposes that some people carry a higher genetic predisposition, or “diathesis,” toward depression, but that predisposition only manifests as actual illness when combined with stressful life events. Studies testing this model have found that the interaction between genetic risk scores and life stress does contribute to depression risk, though the effect is modest. In one large study, the gene-by-environment interaction was strongest in women, where people who reported a high number of stressful events and also carried high genetic risk were substantially more likely to develop depressive symptoms than those with high genetic risk but no recent stress.12PubMed Central. A direct test of the diathesis–stress model for depression
A follow-up validation study in a separate Scottish cohort confirmed these findings while adding nuance. Among participants who reported no stressful life events in the preceding six months, genetic risk scores did not predict depressive symptoms at all. The genetic susceptibility only became visible when stress was present, and its effect scaled with the number of stressful events reported.13Translational Psychiatry. A validation of the diathesis-stress model for depression in Generation Scotland This has real implications for how we think about prevention: reducing environmental stressors may matter most for people with the highest genetic susceptibility, even if we cannot easily identify who those people are in a clinical setting.
Autoimmunity and the HLA Region
Some of the strongest known genetic associations with disease susceptibility involve the HLA region, a stretch of DNA encoding proteins that present bits of foreign material to the immune system so it can decide what to attack. Specific HLA variants have been linked to autoimmune conditions like rheumatoid arthritis, type 1 diabetes, and Graves’ disease for over fifty years.14PubMed Central. The HLA Region and Autoimmune Disease: Associations and Mechanisms of Action The mechanism makes intuitive sense: if the molecules responsible for distinguishing self from non-self are shaped slightly differently, they may occasionally flag the body’s own tissues as threats, triggering an autoimmune attack.
What makes this region especially complicated is its extreme genetic diversity. The HLA genes are among the most variable in the human genome, with strong linkage between nearby variants, meaning they tend to be inherited in blocks. Recent work has focused on how these haplotype blocks affect not just which HLA molecules are present but how much of them appear on cell surfaces, since the quantity of antigen-presenting molecules determines the strength of the immune response against self-tissues.15PubMed. Exploring the HLA complex in autoimmunity: From the risk haplotypes to the modulation of expression Carrying a risk haplotype doesn’t guarantee autoimmune disease, but it loads the dice in ways that other genetic and environmental factors can tip.
Children and Neurotoxic Susceptibility
Susceptibility is not always about catching an infection. Children are substantially more susceptible to the neurotoxic effects of environmental chemicals than adults, and lead is the textbook example. Lead is a systemic toxicant, but it primarily targets the central nervous system, and the developing brain is especially vulnerable.16PubMed Central. Neurotoxic effects and biomarkers of lead exposure: a review Exposure during early life has been linked to cognitive impairment, problems with executive function, abnormal social behavior, and difficulties with fine motor control.17PubMed Central. Cognitive Impairment Induced by Lead Exposure during Lifespan: Mechanisms of Lead Neurotoxicity
The reason children are more susceptible is fundamentally developmental. The process of brain formation, while exquisitely controlled, is also highly labile, meaning it can be disrupted by chemical interference at critical windows that simply do not exist in the adult brain.18PubMed Central. Vulnerability of children and the developing brain to neurotoxic hazards Children also absorb proportionally more lead from their environment due to hand-to-mouth behavior and higher gut absorption rates. The upshot is that a dose of lead that might produce no obvious effects in an adult can cause measurable cognitive harm in a young child.
Susceptibility at the Population Level
When epidemiologists model infectious disease outbreaks, they need to account for the fact that susceptibility is not uniform across a population. Some people are more exposed due to their social connections, some have partial immunity from prior infections or vaccines, and some are genetically less susceptible for reasons like those described above. This heterogeneity has important consequences for herd immunity thresholds, the fraction of a population that needs to be immune to protect the rest.
Classical models that assume everyone is equally susceptible tend to overestimate the threshold needed for herd immunity. When you account for the real-world variation in how susceptible people are, the threshold drops, sometimes substantially. One modeling study showed that the herd immunity threshold predicted by a heterogeneous-population model could be around 40%, compared to 63% from a model assuming a uniform population with the same basic reproduction number.19PubMed Central. Vaccination and herd immunity thresholds in heterogeneous populations However, these lower thresholds come with caveats. When researchers modeled populations as networks rather than well-mixed groups, the advantage of natural infection-induced immunity became less clear, because immune individuals tend to cluster geographically and socially, weakening the protective effect.20PubMed Central. Strength and weakness of disease-induced herd immunity in networks The practical message is that variable susceptibility makes outbreaks harder to predict but also means blanket assumptions about how many people need to be vaccinated can be off in either direction.
Susceptibility to Misinformation and Phishing
The concept of susceptibility extends well beyond biology. In psychology and cybersecurity, researchers study why some people are more likely to fall for false information or social engineering attacks. A large meta-analysis looking at who believes online misinformation found that stronger analytical thinking skills and older age were both associated with better ability to distinguish real news from fake news. Interestingly, though, those same analytical thinkers were more susceptible to believing news that aligned with their ideology, a phenomenon called motivated reflection.21PubMed Central. Susceptibility to online misinformation: A systematic meta-analysis of demographic and psychological factors Being smart does not make you immune to misinformation; it makes you better at rationalizing the misinformation you want to believe.
In the cybersecurity world, phishing simulations at a large European financial organization over two years found that older employees were more susceptible to clicking phishing links, while men were slightly less vulnerable than women. Higher-level employees, who you might expect to be more security-aware, were actually less likely to report phishing emails when they spotted them.22Computer Standards & Interfaces. Human factors in phishing: Understanding susceptibility and resilience The age pattern here runs opposite to the misinformation finding, where older adults were better at spotting fake news. The difference likely reflects distinct cognitive demands: evaluating a news headline draws on accumulated knowledge and skepticism, while recognizing a phishing email requires technical vigilance and familiarity with digital threats.
Heat Vulnerability and Social Determinants
Environmental susceptibility often tracks with social factors as much as biological ones. Research on extreme heat exposure in the United States found that the biggest differences in vulnerability were not simply between age groups but between sociodemographic categories within those age groups. Children and older adults from socially marginalized backgrounds faced greater heat exposure than adults from similar backgrounds, and people of color of any age along with older adults without health insurance emerged as the most exposed groups overall.23PubMed Central. Identifying groups at-risk to extreme heat: Intersections of age, race/ethnicity, and socioeconomic status
A separate mapping study of heat vulnerability identified four key community-level factors: a combined index of poverty, education, race, and green-space availability; social isolation; the prevalence of air conditioning; and the proportion of elderly residents and people with diabetes.24PubMed Central. Mapping community determinants of heat vulnerability These factors explained more than three-quarters of the total variation in heat vulnerability. Biology matters, but so do the neighborhood you live in, whether you have air conditioning, and whether someone checks on you during a heat wave.
Susceptibility Genes in Crop Plants
In agriculture, susceptibility has a surprisingly literal meaning. Plants have what are called susceptibility genes, or S genes, that pathogens exploit to establish infection. Rather than adding new resistance genes to crops (which pathogens can eventually overcome), plant scientists have started disabling these S genes instead. The idea is that removing the molecular handles a pathogen grabs onto provides a form of resistance that is harder for the pathogen to circumvent, because the pathogen would need to find an entirely new entry strategy rather than just evolving around a single resistance protein.25PubMed Central. Stop helping pathogens: engineering plant susceptibility genes for durable resistance
Genome editing tools have made this approach practical across a wide range of crops. S genes have been successfully targeted to improve disease resistance in numerous agricultural systems, with the resulting resistance tending to be broad-spectrum, effective against multiple pathogen types rather than just one.26PubMed. Susceptibility genes in plants: from molecular mechanisms to ecological implications for disease resistance The approach represents a conceptual shift: instead of arming the plant with weapons, you take away the enemy’s foothold.
When Susceptibility Hides Inside a Bacterial Population
Antibiotic susceptibility testing in clinical microbiology assumes that a bacterial isolate is either susceptible or resistant to a given drug. But reality is messier. A phenomenon called heteroresistance describes bacterial populations that appear susceptible overall but contain rare subpopulations capable of growing at much higher antibiotic concentrations. These subpopulations can arise through gene amplification, shifts in plasmid copy number, transposon activity, or purely reversible changes in gene expression, and they represent a hidden form of resistance that standard testing often misses.27PubMed Central. Bacterial heteroresistance mechanisms, dynamics, and emerging diagnostic approaches When a patient is treated based on a susceptibility test that returns “susceptible,” those rare resistant cells can survive, multiply, and cause treatment failure. Heteroresistance is increasingly recognized as a factor in unexplained antibiotic treatment failures, and new diagnostic approaches are being developed to catch it before it becomes a clinical problem.
Stress Corrosion and Material Susceptibility
Even non-living materials have susceptibility profiles. In engineering, stress corrosion cracking describes the failure of metals under the combined influence of mechanical stress and a corrosive environment. Different alloys are susceptible to different combinations of stress and chemistry: steels, aluminum alloys, nickel-based superalloys, and titanium all have distinct vulnerability patterns depending on factors like aqueous chemistry, temperature, pH, and dissolved gases.28PubMed Central. Stress Corrosion Cracking: Mechanisms, Materials Challenges, and Engineering Solutions A pipeline that performs perfectly in one chemical environment can crack and fail in another, not because the steel is “bad” but because the specific combination of stress and environment hits a susceptibility window for that material. Engineers map these susceptibility profiles to choose the right alloy for a given application, in much the same way that physicians try to match treatments to a patient’s individual risk factors.