Once HIV is inside your body, it weakens your immune system by attacking what are called “CD4 cells” or “T cells” in your blood. CD4 cells normally fight infections to keep you healthy, but HIV uses these cells to reproduce and then kills them off. When most of your CD4 cells are killed, you can get sick very easily. Doctors find out how many CD4 cells you have by taking a blood test. The results are your “CD4 count.” CD4 count gives you a good idea of how strong your immune system is. The higher your CD4 count, the healthier your immune system is — and the healthier you are. When your CD4 count falls below 200, you are at a much greater risk of getting sick.

Doctors can also measure how much HIV is in your body with a blood test. The results are your “viral load.” Your viral load can give you an idea of how fast HIV is damaging your immune system, so it can help to predict your future health. People with a high viral load are much more likely to get sick than people with a low viral load. So, the lower your viral load, the better. But even with a very low viral load or one that is called “undetectable,” you still have HIV and can infect someone else.

If HIV weakens your immune system, you will become vulnerable to a variety of HIV-related illnesses: cancers, fungal infections, bacterial infections and parasites. The best way to prevent these illnesses is to watch your immune system carefully and keep it healthy.

Your doctor should test your CD4 count and viral load every 3 to 6 months. These counts will help your doctor determine whether you need HIV treatment. Having drugs in your system may affect both your viral load and your CD4 count. For the most accurate results, try not to use drugs a few days before your tests. This isn’t to hide your drug use, but to get the most accurate test results.

If you feel sick: is it the drugs or HIV?

If you have HIV and you use drugs, it can be hard to tell the difference between problems related to drug use and signs of HIV infection or related illnesses, such as hepatitis, tuberculosis (TB) or a sexually transmitted disease. Symptoms of HIV or HIV-related illnesses — such as fever, weight loss, tiredness and diarrhea — may also be caused by drug use or withdrawal. So you might miss the early signs of an infection that needs treatment.

To get the most accurate diagnosis, try to talk with your doctor honestly about your drug use.