context: Pneumocystis pneumonia ( PCP) is a serious infection that causes inflammation and fluid buildup in your lungs. It's caused by a fungus called Pneumocystis jiroveci that's likely spread through the air. This fungus is very common. Most people have successfully fought it by the time they're 3 or 4 years old. PCP isn't hard to prevent. A healthy immune system can easily control it. But it can make people with weakened immune systems, such as someone with HIV, very sick. People who've gotten an organ transplant, with blood cancers, or who take drugs for autoimmume diseases such as rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis could get it, too. Although it's rare, PCP can also affect other parts of your body, including lymph nodes, liver, and bone marrow. Before we had medicine to treat HIV, about 3/4 of HIV-positive people who developed AIDS got PCP. Antiretroviral therapy ( ART) has prevented HIV-infected people from developing AIDS, and among persons who have developed AIDS, additional preventive therapy has brought this number way down. However, PCP is still the most common opportunistic infection in persons who get AIDS. You're most likely to get PCP when your CD4 cell count is less than 200. PCP patients with HIV are eight times more likely to be hospitalized. People with AIDS can die from it, even though they get treatment. At first, PCP may cause only mild symptoms or none at all. Fever (it's usually low-grade if you have HIV, higher temperature if you don't) Dry cough or wheezing Shortness of breath, especially when you're active Fatigue Chest pain when you breathe Call your doctor if you have these symptoms and HIV or a weakened immune system, because PCP can be fatal. Usually, a lab technician will look at the fluid or tissue from your lungs with a microscope to find traces of the fungus. Your doctor will get a sample by helping you cough up stuff or by using a special tool called a bronchoscope that goes into your airways through your mouth. Or your doctor can do a biopsy, taking a needle or a knife to remove a tiny bit of cells from your lung. A test called PCR (polymerase chain reaction) makes copies of specific pieces of DNA, so it can find smaller amounts of the fungus in samples. You might also get a chest X-ray, or blood tests to check for low oxygen levels. Most often, doctors prescribe a combination of two antibiotics, trimethoprim and sulfamethoxazole, or TMP/ SMX ( Bactrim, Cotrim, or Septra). Depending how sick you are, you'll get this in pills or through a needle in your vein (by IV) at the hospital. Other drugs your doctor might prescribe to fight the infection include: Dapsone ( Aczone), sometimes with pyrimethamine ( Daraprim) Pentamidine ( Nubu Pent, Pentam) that you breathe in through a machine called a nebulizer, maybe in a doctor's office or a clinic ( You could also get a shot if your infection is serious.) Atovaquone ( Mepron) in a liquid that you take with food Corticosteroids can help when your PCP is moderate to severe and you have low oxygen levels. There's no vaccine to prevent this type of pneumonia. The best way to prevent PCP when you have HIV is to keep up with your ART, because it raises your CD4 count. Before your CD4 cell count drops or you get sick, you can also take the same medication that treats PCP, but with different doses and timing. Your doctor may recommend this when: You've had PCP before. Your CD4 count is below 200.. You're taking drugs that suppress your immune system. If you get PCP, your doctor may want you to keep taking medicine after your PCP clears up so you don't get it again. When your CD4 count goes over 200 and stays there for at least 3 months, it may be OK to stop.