Pneumocystis pneumonia (PCP) is a fungal infection in one or both lungs. It is common in people who have a weak immune system, such as people who have AIDS.

The disease is less common in the U.S. than it used to be. But it is still a leading cause of significant infection in people with weak immunity. When it happens, you need medical care right away.

PCP is caused by a fungus called Pneumocystis jirovecii. Many people live with this fungus in their lungs every day. It's common all over the world. It often causes little to no trouble for people with healthy immune systems.

But if your immune system is weakened, you have a greater chance of getting PCP. Your immune system may be weakened by HIV or AIDS, cancer, organ transplant, medicines that suppress the immune system, or another condition that causes the immune system to not work well. PCP takes advantage of your weak immune system to attack your lungs.

If not treated right away, PCP can be severe and even fatal.

You are more likely to get PCP if you have a weakened immune system. This can be caused by:

• HIV and AIDS.
• Taking immunosuppressant medicines, including corticosteroids.
• Cancer and its treatment.
• Chronic lung diseases.
• Organ transplant or hematopoietic cell transplant.
• Autoimmune diseases such as lupus or rheumatoid arthritis.

Symptoms of PCP may happen suddenly. Or they may develop over a few weeks or months. The most common symptoms to watch for include:

• A fever that comes on suddenly.
• A cough.
• Trouble breathing, which often gets worse with activity.
• A dry cough, with little or no mucus.
• Chest tightness.
• Weight loss.
• Night sweats.
• Extreme tiredness (fatigue).

If you have any of these symptoms and think you could have PCP, especially if you have a condition that suppresses the immune system, see your doctor right away.

Your doctor can diagnose PCP based on your health history and a physical exam. You may need these tests:

• Chest X-ray. This test uses a small amount of radiation to make images of internal tissues, bones, and organs, including the lungs.
• Blood tests. Your doctor may do blood tests to see if you have an infection and if it has spread to the blood. They may also do an arterial blood gas test to check the amount of oxygen in your blood.
• Sputum culture. This test is done on the material that is coughed up from the lungs and into the mouth. A sputum culture is often used to test for the PCP fungus in your lungs.
• Bronchoscopy. This is a direct exam of the main airways of the lungs (bronchi) using a flexible tube (bronchoscope). It's often done with a procedure called a lavage. This washes out some of the liquid in the lung so it can be checked under a microscope by a doctor. A small sample of lung tissue (a biopsy) may be taken and checked in a lab.

If you have severe PCP, your doctor will likely treat you in a hospital. You will get an I.V. (intravenous) medicine that is a combination of two antibiotics. They are trimethoprim (TMP) and sulfamethoxazole (SMX).

Other medicines are available to treat the condition in cases where TMP/SMX cannot be given (such as sulfa allergies). As you get better, you will likely be able to switch to antibiotics in a pill form.

If you have a disease that weakens your immune system, your doctor will check your blood count regularly to see how strong your immune system is. If you have a weak immune system, your doctor may give you medicine to prevent PCP before it happens.

Smokers are also at a greater risk of getting PCP. If you smoke or use vaping devices, quitting will make your lungs healthier. It will also help keep you from getting lung infections like PCP.

The best way to prevent PCP if you have a weak immune system is to get regular blood tests and take preventive medicines when needed. Flu and pneumococcal vaccines prevent people from getting certain types of pneumonia. But they don't prevent PCP. In addition, people with weakened immune systems may not be candidates for their use. Talk with your doctor about vaccines and which ones may be right for you.

• Pneumocystis pneumonia (PCP) is an infection in one or both of the lungs caused by a fungus.
• A weak immune system puts a person at risk for PCP.
• The most common symptoms of PCP are sudden start of fever, cough, trouble breathing that often gets worse with activity, dry cough with little or no mucus, extreme tiredness, and chest discomfort.
• If you have symptoms of PCP, see your doctor right away.
• Severe PCP is often treated in a hospital with antibiotics given through an I.V.
• Get regular blood tests and take prescribed preventive medicine if you have a weak immune system. Ask your doctor which vaccines may be right for you.

Tips to help you get the most from a visit to your doctor:

• Know the reason for your visit and what health issues you need to get addressed.
• Before your visit, write down questions you want answered.
• Bring someone with you to help you ask questions and remember what your doctor tells you.
• At the visit, write down the name of a new diagnosis and any new medicines, treatments, or tests. Also, write down any new instructions your doctor gives you.
• Know why a new medicine or treatment is prescribed and how it will help you. Also, know what the side effects are and when they should be reported.
• Ask if your condition can be treated in other ways.
• Know why a test or procedure is recommended and what the results could mean.
• Know what to expect if you do not take the medicine or have the test or procedure.
• If you have a follow-up appointment, write down the date, time, and purpose for that visit.
• Know how you can contact your doctor if you have questions, including after office hours and on weekends and holidays.