When someone gets sick more often than normal, the broad medical term is immunocompromised, meaning the immune system is weakened and less able to fight off infections. Depending on the cause, a doctor might use more specific terms like immunodeficiency or simply describe it as having lowered immune function. Adults typically get two to three colds per year, and children often get more. If you’re consistently exceeding that range or catching infections that linger and worsen, something deeper may be going on.
Immunodeficiency: Primary vs. Secondary
Immunodeficiency comes in two forms. Primary immunodeficiency is inherited, caused by gene mutations passed from parents to children. These conditions are relatively rare and usually show up early in life, though some aren’t diagnosed until adulthood. Secondary immunodeficiency is far more common and develops later due to outside factors: chronic illnesses like diabetes or kidney disease, infections like HIV, medications like corticosteroids, or organ transplantation. The key difference is that secondary immunodeficiency can sometimes be reversed once the underlying cause is treated or removed.
Red Flags That Go Beyond “Normal” Sick
Everyone catches colds. The question is when frequent illness crosses the line into something that deserves medical attention. Clinical warning signs originally developed for identifying primary immunodeficiency, but useful for spotting secondary immunodeficiency too, include four or more ear infections in a single year, two or more sinus infections in a year, two or more deep tissue infections (including bloodstream infections), and recurrent deep skin or organ abscesses. Chronic diarrhea that won’t resolve is another signal that sometimes appears even when other warning signs are absent.
If infections keep coming back, take unusually long to clear, or require repeated courses of antibiotics, those patterns are worth bringing to a doctor. The issue isn’t necessarily a single bad cold season. It’s a pattern of infections that are more frequent, more severe, or harder to shake than what you’d expect.
How Stress Quietly Weakens Your Defenses
Cortisol, the hormone your body releases during stress, is designed to be anti-inflammatory in short bursts. It temporarily dials down the immune response so your body can focus on the immediate threat. That’s fine for a stressful afternoon. But when stress is chronic, cortisol stays elevated, and the immune system starts to malfunction. Immune cells carry receptors for stress hormones, and prolonged exposure makes them less responsive over time. Eventually, the system becomes “resistant” to cortisol’s normal regulatory effects, leading to an accumulation of stress hormones and increased inflammatory signaling that further compromises immune function.
This is one reason people under sustained pressure at work, in caregiving roles, or dealing with ongoing emotional strain tend to catch everything going around. The connection between stress and illness isn’t just psychological. It’s a measurable hormonal disruption.
Sleep Has a Bigger Impact Than You Think
People who sleep five hours or fewer per night are 44% more likely to catch a cold compared to those sleeping seven to eight hours. They’re also nearly 40% more likely to develop pneumonia. Even sleeping too much (nine or more hours) raises cold risk by about 20%, suggesting that disrupted or poor-quality sleep in either direction is a problem.
One particularly telling study exposed 164 healthy volunteers to a cold virus after tracking their sleep for a week. Those who slept less were significantly more likely to actually develop the cold, not just report symptoms. The effect holds up even with vaccination: short sleepers who received two doses of an mRNA COVID vaccine still had a 56% higher risk of prolonged post-infection symptoms compared to normal sleepers. Sleep isn’t just rest. It’s active immune maintenance.
Nutritional Gaps That Lower Immunity
Your immune cells depend on a steady supply of specific nutrients to function. Zinc deficiency alone reduces the killing power of natural killer cells, weakens the ability of neutrophils (the most abundant white blood cells and the first responders to infection) to engulf pathogens, and increases the death rate of B cells that produce antibodies. Low levels of protein, folate, and the mineral selenium cause similar breakdowns in immune cell production and signaling.
Malnutrition doesn’t have to mean starvation. Older adults with low protein intake show measurable declines in immune cell activity, including reduced ability of white blood cells to multiply when needed and weakened function of cells that physically consume invaders. Supplementing with zinc, selenium, and key vitamins has been shown to increase protective immune cell counts in older adults and improve their antibody response to flu vaccines.
Your Gut Houses 70% of Your Immune System
About 70% of the immune system is located in the gut, where it interacts constantly with trillions of bacteria. The diversity and composition of those bacteria directly influence how well immune cells function. A fiber-rich diet supports a varied microbiome and reduces inflammatory responses. The typical Western diet, heavy in processed foods, sugar, saturated fat, and animal protein, does the opposite: it narrows bacterial diversity and promotes chronic low-grade inflammation that diverts immune resources away from fighting infections.
This means that someone eating poorly may not have a genetic immune problem at all. Their immune system is simply underfed and working in an inflammatory environment. Shifting toward more plant fiber, fermented foods, and dietary variety can measurably improve the gut’s microbial balance and, by extension, immune readiness.
Environmental Exposures That Mimic Weak Immunity
Sometimes the issue isn’t your immune system but your environment. Chronic mold exposure is a prime example. People living or working in moldy buildings frequently report fatigue, pain, brain fog, anxiety, and what feels like being perpetually sick. These symptoms are nearly identical to the “sickness behavior” your body produces during a bacterial or viral infection, because the underlying mechanism is similar: inhaled mold spores trigger a strong innate immune response in the lungs, releasing inflammatory molecules called cytokines. Those cytokines then activate immune cells in the brain, producing the same malaise, fatigue, and withdrawal you feel when fighting the flu.
Even non-toxic, non-viable mold spores can trigger this response. The immune system reacts to mold fragments themselves, not just living mold or its toxins. So someone exposed to mold daily may feel chronically ill without ever testing positive for a traditional infection or immune disorder. If your frequent illness tracks with a specific building or living space, air quality is worth investigating.
What Testing Looks Like
If your doctor suspects an immune issue, the first step is usually a complete blood count with differential, which measures your levels of different white blood cell types. From there, testing can include measuring immunoglobulin levels (the antibodies IgG, IgA, and IgM) to see if your body produces enough of them. Doctors can also check how well your immune system responds to vaccines you’ve already received by measuring antibody levels against specific diseases like tetanus or pneumococcal bacteria. A normal response shows a meaningful increase in those antibodies after vaccination. A weak or absent response suggests the immune system isn’t learning from exposures the way it should.
These tests are straightforward blood draws. They help distinguish between someone who’s getting sick frequently due to lifestyle or environmental factors and someone with a genuine immunodeficiency that needs targeted treatment.