Opportunistic Infections: What They Are, Who Gets Them, and How to Stay Safe

When your immune system is down, normally harmless germs can turn dangerous. These are called opportunistic infections, infections that take advantage of a weakened immune system to cause illness. Also known as OIs, they don’t normally hurt healthy people — but for someone on chemotherapy, living with HIV, or taking drugs like rituximab or mycophenolate, they can be life-threatening. Think of your immune system like a security guard. When it’s working, it keeps out troublemakers. When it’s tired, injured, or silenced by medication, those troublemakers walk right in.

Common triggers for opportunistic infections, infections that take advantage of a weakened immune system to cause illness include immunosuppressants, medications that intentionally lower immune activity to prevent organ rejection or control autoimmune diseases — drugs like methotrexate or corticosteroids. People with HIV, a virus that attacks the immune system, particularly CD4 cells are especially vulnerable, especially when their CD4 count drops below 200. Cancer patients on chemotherapy, powerful drugs that kill fast-growing cells, including immune cells are another high-risk group. Even people on long-term steroids or biologics for arthritis or psoriasis can develop these infections. They’re not rare — they’re predictable, and that’s why prevention matters more than cure.

These infections come in many forms: fungal like candidiasis or pneumocystis pneumonia, viral like CMV or herpes, bacterial like tuberculosis. Some hide in your lungs, others in your gut or skin. Many show up as persistent cough, fever that won’t quit, mouth sores, or unexplained weight loss. The good news? You don’t have to wait for symptoms to show. Testing, vaccines (when timed right), and careful hygiene can block many before they start. Knowing which meds weaken your defenses — and how to talk to your doctor about it — is your first line of defense.

Below, you’ll find real, practical advice from people who’ve been there — how to spot early signs, what drugs to avoid, when to get tested, and how to protect yourself while managing chronic conditions. No fluff. Just what works.