Being immunocompromised means your immune system has a reduced ability to fight infections and other diseases. This can happen because of a genetic condition you were born with, a chronic illness that developed over time, medications you take, or even the natural process of aging. Some people are mildly immunocompromised and may not realize it, while others face severe immune deficiency that requires constant vigilance against infection.
Genetic Conditions That Weaken Immunity
Some people are born with immune systems that don’t function properly. These are called primary immunodeficiencies, and they affect different parts of the immune system depending on the specific genetic defect. There are hundreds of these conditions, ranging from mild to life-threatening.
Selective IgA deficiency is one of the most common. Your body doesn’t produce enough of a specific type of antibody, which leads to frequent sinus and lung infections. Common variable immunodeficiency (CVID) is similar but broader, affecting multiple types of antibodies and increasing the risk of both infections and autoimmune disease. Many people with these conditions aren’t diagnosed until adulthood, after years of unusually frequent infections.
On the severe end of the spectrum, severe combined immunodeficiency (SCID) leaves infants with almost no functional immune defense. Without treatment, babies with SCID develop life-threatening infections within their first months. Other genetic conditions like DiGeorge syndrome and Wiskott-Aldrich syndrome affect the immune system alongside other body systems, causing a mix of symptoms that can include heart defects, bleeding problems, or difficulty with movement.
Chronic Diseases That Suppress Your Defenses
Many common chronic illnesses gradually erode immune function as a secondary effect. HIV is the most well-known example. The virus directly attacks the immune cells responsible for coordinating your body’s defense against pathogens. Without treatment, HIV progresses to AIDS, a state of severe immunodeficiency.
Diabetes is a less obvious but significant cause. Persistently high blood sugar impairs the ability of white blood cells to reach infection sites and kill pathogens effectively. Chronic kidney disease and liver disease both compromise immunity because these organs play key roles in filtering waste and producing immune proteins. When they fail, the whole system weakens. Severe or prolonged malnutrition, including protein deficiency, also qualifies as a cause of secondary immunodeficiency because the body simply lacks the raw materials to build immune cells and antibodies.
Certain cancers directly attack the immune system. Leukemia and lymphoma are cancers of blood cells and lymph tissue, meaning the immune system itself is what’s diseased. Even cancers that don’t originate in the immune system can suppress it by spreading to bone marrow or by consuming resources the body needs for immune function.
Medications That Lower Immune Function
Medications are one of the most common reasons people become immunocompromised, and this is almost always intentional. Doctors suppress the immune system on purpose when it’s causing harm, such as attacking a transplanted organ or driving an autoimmune disease.
Corticosteroids
Prednisone and similar steroids are widely prescribed for conditions like asthma, lupus, and inflammatory bowel disease. At low doses or for short courses, they don’t significantly suppress immunity. The threshold that clinical guidelines use is 20 mg or more of prednisone daily (or 2 mg per kilogram of body weight per day) for longer than 14 days. Above that level, your immune response is meaningfully weakened, and after stopping, it takes at least a month for your system to recover enough to respond normally to vaccines.
Chemotherapy and Radiation
Cancer treatments are designed to kill rapidly dividing cells, which includes cancer cells but also the cells in your bone marrow that produce white blood cells. This is called bone marrow suppression. When your bone marrow can’t keep up with white blood cell production, your neutrophil count drops. Neutrophils are the frontline cells that destroy bacteria and fungi. A normal neutrophil count is above 1,500 per microliter of blood. Below 1,000 is moderate neutropenia, and below 500 is severe, meaning your body has very limited ability to fight off even common infections.
Anti-Rejection Drugs After Transplant
Organ transplant recipients take immunosuppressive medications for the rest of their lives to prevent their body from rejecting the new organ. The first three months after transplant carry the highest risk, because the medication regimen is most intense during that period. Over time, doses are typically reduced, but transplant recipients remain immunocompromised permanently. The CDC specifically lists solid organ transplant recipients on anti-rejection medication as a group that may not be fully protected by vaccines.
Biologics for Autoimmune Diseases
A newer class of drugs targets specific molecules in the immune system rather than suppressing it broadly. TNF inhibitors, for example, are prescribed for rheumatoid arthritis, Crohn’s disease, psoriasis, and several other autoimmune conditions. These drugs block a key signaling protein that drives inflammation, but that same protein is also involved in fighting infections. More than 10% of people taking TNF inhibitors experience upper respiratory infections, sinusitis, or other common infections. Serious infections, including bacterial, fungal, and viral, can also occur and in rare cases are fatal.
Aging and the Immune System
The immune system deteriorates with age in a process called immunosenescence. This isn’t a disease or a condition. It happens to everyone, though the pace varies based on genetics, diet, physical activity, and overall health.
As you age, your body produces fewer of the immune cells that recognize new threats, and the cells it does produce work less efficiently. The germ-eating cells that serve as first responders become slower and less effective at their job. Cells that present foreign invaders to the rest of the immune system decline in both number and function. Meanwhile, the body ramps up a low-grade, chronic inflammatory state that paradoxically coexists with weaker defenses against actual pathogens. This is why older adults are more vulnerable to infections like influenza and pneumonia and why vaccines tend to produce weaker, less durable immunity in people over 65.
How to Know If You’re Immunocompromised
There’s no single test that labels someone immunocompromised. Instead, the determination comes from your overall medical picture: your diagnoses, the medications you take, and sometimes specific blood work. A low white blood cell count or low antibody levels can confirm immune suppression, but many people are considered immunocompromised based on their treatment alone, without needing a blood test to prove it.
The CDC considers you moderately to severely immunocompromised if you are actively receiving chemotherapy, have had a solid organ transplant and take anti-rejection drugs, have a primary immunodeficiency, or use high-dose corticosteroids for extended periods. If you fall into any of these categories, you may need additional vaccine doses because your body’s response to standard vaccination schedules can be insufficient.
People with milder forms of immune compromise, such as well-controlled diabetes or age-related immune decline, may not meet the formal threshold but still face higher infection risk than the general population. The distinction matters most for vaccination schedules and for knowing when to take extra precautions during outbreaks of respiratory illness or other infectious diseases.