An opportunistic infection (OI) is the name given to an HIV-related illness. This is often something that your body would normally fight off, but that takes advantage of the damage HIV has caused to your immune system.

The lower the CD4 count, the higher the risk of HIV-related illnesses. This is why monitoring of CD4 levels is important when you are not on treatment.

You can still feel well and healthy with a low CD4 count. Even when this is below 200, below 100, below 50 or even below 10. The risk of serious health-related problems increases as the CD4 count drops. All guidelines recommend treatment for someone with a count below 200 and most now recommend treating at 350.

Different illnesses become more likely at different CD4 counts. Many serious and life threatening illnesses become a risk when the CD4 count drops to under 200 cells/mm³.

Opportunistic infections that occur at different CD4 counts

CD4 count below 300

• Diarrhoea from bacteria infections microsporidia and cryptosporidia
• Skin problems – candida (thrush), dry skin, etc

CD4 count below 200

• PCP (fungal pneumonia) and chest infections
• Toxoplasmosis – parasitic infection that can cause brain lesions

CD4 count below 100

• MAI /MAC (mycobacterium avium complex) – bacterial infections similar to TB
• Cryptococcus – fungal infection that can cause meningitis in the brain and PCP-like symptoms in the lungs

CD4 count below 50

• CMV (cytomegalovirus) – a viral infection that can cause permanent vision loss and blindness and can also affect other organs

Information on these infections are included in the OI chapter of this resource.

• The lower your CD4 count the higher the risk of these and other illnesses.
• When your CD4 count increases after starting HIV treatment, your immune system is often able to deal with these infections by itself again.
• Treatment to prevent OIs (primary or secondary prophylaxis) can usually be stopped afted the CD4 count increases to safer levels.