Preventing Opportunistic Infections (Prophylaxis)

When your immune system is significantly weakened by HIV, you become vulnerable to opportunistic infections (OIs). These are illnesses caused by germs—like viruses, bacteria, fungi, or parasites—that usually don’t cause trouble in people with healthy immune systems but take the “opportunity” to cause disease when the body’s defenses are down ^[1][2].

Preventing these infections is a cornerstone of HIV care, especially if you have a low CD4 count. This prevention is called prophylaxis ^[3].

Common Opportunistic Infections

Several specific infections are closely watched in people with advanced HIV:

• Pneumocystis jirovecii pneumonia (PCP): A serious fungal lung infection that is the most common OI in the U.S. ^[1][4].
• Toxoplasmic encephalitis (Toxoplasmosis): An infection caused by a parasite that can affect the brain, often triggered by a reactivation of a previous infection ^[5].
• Mycobacterium avium complex (MAC): A bacterial infection that can spread throughout the body, typically only seen in people with very low CD4 counts ^[6].
• Candidiasis: A yeast infection (thrush) that can affect the mouth, throat, or esophagus ^[1].
• Tuberculosis (TB): A bacterial infection that primarily affects the lungs but can spread elsewhere ^[1]. You will be tested for latent TB using an IGRA blood test or a skin test; if positive, you will be evaluated for active TB and treated for latent TB as appropriate. Be aware that rifamycin-based TB drugs have major interactions with ART.

CD4 Thresholds for Starting Prophylaxis

Doctors use your CD4 count as a guide to know when to start “preventive” medications. The goal is to start these medicines before you get sick ^[7].

• CD4 count below 200 cells/µL, CD4 percentage below 14%, or CD4 100-200 with detectable HIV RNA: At this level, you will likely start prophylaxis for PCP ^[7][4].
• CD4 count below 100 cells/µL (with positive Toxoplasma IgG antibodies): If you have evidence of prior exposure to the Toxoplasma parasite, your doctor will start prophylaxis for Toxoplasmosis ^[5].
• CD4 count below 50 cells/µL: Primary prophylaxis is generally not recommended if you are immediately starting effective ART. When ART is delayed or not effective, it may be considered for starting MAC prophylaxis ^[6][8].

Common Prophylaxis Medications

The most important drug in OI prevention is trimethoprim-sulfamethoxazole (TMP-SMX), also known by brand names like Bactrim or Septra ^[3].

• TMP-SMX: This is the first-choice medication for preventing both PCP and toxoplasmosis ^[3][5].
• Alternatives: If you have a sulfa allergy or cannot tolerate TMP-SMX, your doctor may prescribe dapsone, atovaquone, or inhaled pentamidine ^[3][9]. Note that aerosolized pentamidine prevents PCP but not toxoplasmosis, and dapsone requires G6PD testing and must be combined with pyrimethamine/leucovorin when toxoplasmosis coverage is needed.
• MAC Prevention: If needed, MAC is usually prevented with antibiotics like azithromycin or clarithromycin ^[6].

Medical Emergency Warning: If you develop a severe rash (especially with fever, blistering, peeling, mouth sores, or breathing trouble) while taking TMP-SMX or another prophylactic drug, seek immediate medical advice.

When Can You Stop Prophylaxis?

The best way to stop taking these extra pills is to strengthen your immune system with Antiretroviral Therapy (ART) ^[1]. As ART controls the virus, your CD4 count will begin to rise.

You can typically stop prophylaxis when:

1. Your CD4 count rises: For PCP and Toxoplasmosis, this usually means your count stays above 200 cells/µL for at least 3 to 6 months ^[10][11].
2. Viral Suppression is achieved: If your viral load is “undetectable,” some doctors may consider stopping prophylaxis earlier—for example, if your CD4 count is between 100 and 200 cells/µL—but this is a specialized decision made based on your specific health history ^[12][10]. Note that discontinuation and restart criteria differ specifically for each infection and depending on whether it is primary or secondary prophylaxis. An undetectable viral load does not remove OI risk if the CD4 count remains very low.

Never stop taking your prophylaxis medication without your doctor’s approval. If your CD4 count drops again in the future, you may need to restart these preventive treatments to keep your immune system safe ^[13]. In addition to pills, discuss an individualized vaccination plan with your doctor and practice everyday exposure advice like food/water safety and precautions around soil or animal waste.