Signs and symptoms

Pneumonia can be tricky. It often mimics a cold or the flu, so you may not realize you have a more serious condition. In addition, signs and symptoms of pneumonia vary greatly, depending on any underlying conditions you may have and the type of organism causing the infection:

• Bacteria. Dozens of types of bacteria can cause pneumonia. Bacterial pneumonia can occur on its own, or you may develop it after you've had an upper respiratory infection such as a cold or the flu. Signs and symptoms, which are likely to come on suddenly, include shaking chills, a high fever, sweating, chest pain (pleurisy) and a cough that produces thick, greenish or yellow phlegm. If you're an older adult or have a chronic illness, you may have fewer or milder symptoms. Still, don't treat pneumonia lightly. For people age 65 and older, or those with a chronic illness, pneumonia can be extremely serious.

• Viruses. About a dozen different viruses — including the same viruses that cause influenza — are responsible for half of all cases of pneumonia. Viral pneumonia strikes primarily in the fall and winter and tends to be more serious in people with cardiovascular or lung disease. It usually starts with a dry (nonproductive) cough, headache, fever, muscle pain and fatigue. As the pneumonia progresses, you may become breathless and develop a cough that produces phlegm. When you have viral pneumonia you run the risk of also developing a secondary bacterial pneumonia.

• Mycoplasma. This tiny organism causes symptoms similar to those of both bacterial and viral infections, although the symptoms appear more gradually and are often milder than are those of other kinds of pneumonia. If you've been told you have "walking pneumonia," it's probably caused by mycoplasma. You may not be sick enough to stay in bed or to seek medical care. In fact, you may never know you've had pneumonia. Mycoplasma pneumonia spreads easily in situations where people congregate and is common in child-care centers and among school children and young adults. Although not bacterial, mycoplasma pneumonia responds well to treatment with the appropriate antibiotics.

• Chlamydia. This bacterium causes symptoms similar to those of mycoplasma pneumonia. Although everyone is at risk, chlamydia pneumonia is most common in school-age children, and as many as half of Americans are likely to have been infected by the time they turn 20.

• Fungi. Certain types of fungus also can cause pneumonia, especially Histoplasma capsulatum, which is common in the Mississippi and Ohio River valleys. Most people experience few if any symptoms after inhaling this fungus, but some develop symptoms of acute pneumonia, and still others may develop a chronic pneumonia that persists for months.

• Pneumocystis carinii. Pneumonia caused by P. carinii is the most common opportunistic infection affecting Americans with AIDS. People whose immune systems are compromised by treatment with corticosteroids, organ transplants or cancer also are at risk. The signs and symptoms of Pneumocystis carinii pneumonia (PCP) include a cough that doesn't go away, fever and trouble breathing. In the past, P. carinii was considered a type of parasite, but more recent studies suggest that this microorganism is more closely related to fungi.

Causes

Your lungs are two spongy organs surrounded by a moist membrane (the pleura). When you inhale, air is carried through the windpipe (trachea) to your lungs in two major airways called bronchi. Inside your lungs, the bronchi subdivide nearly 20 times into a million smaller airways (bronchioles), which finally end in clusters of tiny air sacs called alveoli.

At each stage of this process, there are mechanisms to protect your lungs from infection. In fact, you're frequently exposed to bacteria and viruses that can cause pneumonia, but your body normally keeps them from entering your lungs and causing a problem. But sometimes — for reasons that aren't always well understood — these microorganisms can get past your body's defenses.

Pneumonia is sometimes classified according to where or how you're exposed to the disease:

• Community-acquired pneumonia. This refers to pneumonia you acquire in the course of your daily life — at school, work or the gym, for instance.

• Hospital-acquired (nosocomial) pneumonia. If you're hospitalized, you're much more likely to develop pneumonia, especially if you are on a mechanical ventilator, are in the intensive care unit or have a compromised immune system. This type of pneumonia can be extremely serious, especially for older adults, young children and people living with HIV/AIDS. Unfortunately, it remains a common problem in spite of stringent efforts to control it.

• Aspiration pneumonia. This type of pneumonia occurs when foreign matter is inhaled (aspirated) into your lungs — most commonly when the contents of your stomach enter your lungs after you vomit. This most often happens when a brain injury or other condition affects your normal gag reflex.

• Pneumonia caused by opportunistic organisms. This type of pneumonia strikes people with compromised immune systems. Organisms that aren't harmful for healthy people can be extremely dangerous for those with AIDS, sickle cell disease and other conditions that impair the immune system. For example, P. carinii pneumonia almost never occurs in otherwise healthy people. Medications that suppress your immune system, such as corticosteroids or chemotherapy for cancers such as leukemia and lymphoma, also can put you at risk of opportunistic pneumonia.

Risk factors

You're at increased risk of pneumonia if you're age 65 or older. Very young children, whose immune systems aren't fully developed, also are at risk of pneumonia. You're also more likely to develop pneumonia if you:

• Have certain diseases. These include immune deficiency diseases such as HIV/AIDS and chronic illnesses such as cardiovascular disease, emphysema or diabetes. You're also at increased risk if you've had your spleen removed, or your immune system has been impaired by chemotherapy or long-term use of immunosuppressant drugs.

• Smoke, or abuse alcohol or drugs. Smoking damages your airways, and alcohol interferes with the action of white blood cells that fight infection. If you inject illegal drugs, there's a chance you may develop injection-site infections that can travel through your bloodstream to your lungs.

• Are exposed to certain chemicals or pollutants. Your risk of developing some uncommon types of pneumonia increases if you work in agriculture, construction or around certain industrial chemicals. Exposure to air pollution or toxic fumes can also contribute to lung inflammation.

• Live in certain parts of the country. Two types of fungus that occur in the soil in certain parts can cause lung infections and pneumonia. Coccidioidomycosis, for example, is widespread throughout Southern California and the desert Southwest. The majority of people exposed to the fungus don't get sick, but a few develop severe pneumonia.

• Histoplasmosis is a serious lung infection caused by a soil-borne fungus that's most prevalent in the Ohio and Mississippi River valleys. Infants, young children, older adults and people with chronic lung disease or HIV/AIDS are at increased risk of severe symptoms.