A week into the worst case of flu she'd ever had, Donna Jones was still accumulating symptoms.

Fever, chills, fatigue, nausea -- those had been there from the start. Then a deep, raucous cough with greenish sputum. Eyes drawn and watery. Pain in the chest. A bad taste in her mouth. No energy, no appetite, no getting better.

"Absolutely lousy" is how she remembers it. She couldn't stand the thought of food, not even old standbys like chicken-noodle soup or toast or flat Coke. Fresh orange juice tasted rancid. "Cherry 7-Up was the only thing I could tolerate -- with an occasional oyster cracker," recalled Jones, 44, of Silver Spring. "It was the hardest 12 pounds I ever lost in my life."

And then: night sweats, flashes of light even in the dark with her eyes closed. She noticed flecks of blood in her sputum.

Donna Jones is one of those lucky people who never go to the doctor except for routine checkups, but here she was, on the Monday before the inauguration, sitting in a doctor's waiting room for the fourth time in a week, thinking she had "no idea how I could even stand to be there."

She was still in the radiologist's office when her family doctor phoned to report on the chest X-ray. "Donna, you have pneumonia," he told her.

"The word totally devastated me. I was in shock. I thought: Pneumonia kills!"

Not a Single Disease

It can kill -- pneumonia is the sixth-leading cause of death in the United States and fifth among the elderly. But most patients recover, as Donna Jones has, with rest and treatment with antibiotics.

More than 2 million Americans get pneumonia each year, according to the federal Centers for Disease Control and Prevention. About 500,000 are hospitalized; 50,000 to 75,000 die.

Pneumonia is an acute infection of the lung. Not the airways to the lung (as in bronchitis), not the nose and throat (as in a common cold), but the lung tissue itself -- the millions of air sacs deep inside the pair of spongy balloons that enable humans to breathe. If too many of those air sacs are infected and inflamed, the lungs can't do their job of exchanging carbon dioxide waste for fresh oxygen to nourish the blood.

It shows up on a chest X-ray as a white cloud against the dark background of the healthy lung. The cloudy portion represents air sacs that no longer contain air -- because they are filled with fluid and inflammatory white blood cells from the body's infection-fighting response.

Pneumonia is not a single disease. It is an illness -- a kind of pulmonary damage -- with many causes. (Going out into the cold isn't one of them, despite the time-honored parental warning.) Pneumonia is much more common in the winter and early spring, but that's because many cases develop from influenza, which flourishes in winter months.

"The cold itself, by itself, will not cause pneumonia," said Octavius D. Polk Jr., a pulmonary specialist at Howard University Medical Center and Providence Hospital.

More than 100 different microbes -- microscopic "bugs" -- have been found to cause infections that lead to pneumonia. These include a variety of bacteria (which respond to antibiotics), viruses (which don't), parasites, fungi and mycoplasma, which resemble bacteria but are even tinier.

"We're talking about a list that's several phone-book pages long," said Frederick L. Ruben, professor of medicine at the University of Pittsburgh School of Medicine and a lung specialist at Montefiore University Hospital in Pittsburgh.

That's why the term "pneumonia" by itself can be so misleading. It doesn't tell what caused the infection, how severe it is, whether it's contagious (usually it's not) or what might treat it. "It's a bit like saying, 'I've got a sore leg,' " Ruben said.

Pneumonia can come on suddenly, within hours of infection, or take days. It can cause a dry cough or a "productive" cough full of sputum. High fever or low fever. It may mimic the flu or feel quite different.

"Some bacteria cause pneumonia without actually damaging the lung much," Ruben said. "Other bacteria literally chew apart the lungs."

It can be a relatively mild complication -- or turn into a life-threatening illness. Tuberculosis is a form of pneumonia; so is Legionnaire's disease, which takes its name from its most famous outbreak, at an American Legion convention in Philadelphia in 1976. Muppeteer Jim Henson died suddenly three years ago of a rare type of bacterial pneumonia. In earlier times, pneumonia was such a common killer of the elderly that it was given the ironic nickname, "the old man's friend."

Cases are labeled on the basis of where the patient contracted the illness (community-acquired or hospital-acquired), what part of the lung is infected (lobar or bronchial) and what kind of microbe caused the infection (bacterial or viral or mycoplasmal). But the specific terminology gets terribly confusing. A bacterium called Streptococcus pneumoniae causes pneumococcal pneumonia -- the most prevalent type -- but not streptococcal pneumonia. Another common pneumonia bug, Hemophilus influenzae, was named a century ago when it was erroneously thought to cause influenza; it is not related to the type of pneumonia caused by flu.

Pneumonia still strikes mainly the old and the very young -- but can also hit anyone in between whose immune defenses are down because of chronic illness, AIDS, lymphoma, an organ transplant or a bout with the flu.

Like Donna Jones, for example. The influenza she caught last month apparently lowered her resistance, allowing the pneumonia infection to take hold.

Infection's Breeding Ground

The human respiratory system -- mouth, nose, throat, sinuses, airways, lungs -- is a prime breeding ground for bacteria and other microbes that thrive in warm, moist climates.

"Most of the time, a healthy person has adequate defenses against these organisms," said Morgan D. Delaney, a pulmonary specialist and associate professor of medicine and physiology at George Washington University Medical Center. "Pneumonia develops when the host defenses are altered in some way -- from smoking, alcohol abuse, diabetes or HIV, for example -- and the scale is tipped toward the organism overwhelming the defense."

The symptoms vary, but usually include a fever, severe cough, fatigue, painful breathing and the vaguely uneasy lethargy sometimes termed malaise. And sudden chills -- a case of the shivers that doctors sometimes call the "rigors."

"They feel really sick," Delaney said. "They come in saying, 'I feel rotten.' "

Although the symptoms may overlap somewhat with those of a bad cold or ordinary flu, pneumonia is a far more serious illness.

"Usually, the tipoff is the chest pain and shortness of breath," said Xilla Ussery, a pediatrician and epidemiologist in CDC's respiratory disease section in Atlanta.

"The danger signals that say you should see a doctor are shortness of breath, a fever and a cough you didn't have before," Pittsburgh's Ruben said. An additional telltale is the coughed-up sputum -- not just spit or phlegm, as in a routine cold, he said, but "cloudy guck from the lungs."

Another myth is that pneumonia is contagious.

"Actually, it's not easily transmitted, person to person," said Alfred Munzer, director of pulmonary medicine at Washington Adventist Hospital in Takoma Park and president-elect of the American Lung Association. With one key exception -- tuberculosis -- bacterial pneumonia is relatively uncontagious. Viral pneumonias and those caused by mycoplasma are transmitted through droplets in the air and occasionally spread from one family member to another, but they are much less contagious than flu or colds.

Death Rates Rising

Nearly 74,000 Americans died of pneumonia in 1991, according to the National Center for Health Statistics. Only five causes of death killed more: heart disease, cancer, stroke, accidents and injuries, and chronic lung disease. Indeed, pneumonia is often the final complicating illness in patients who die of lung cancer or chronic lung disease.

Worldwide, pneumonia's toll is even more devastating. The United Nations Children's Emergency Fund (UNICEF) estimates that 3.5 million children die of pneumonia every year. The vast majority of those deaths could be prevented, UNICEF officials said, if bacterial pneumonia were diagnosed promptly and treated with antibiotics.

As recently as the late 1930s, before penicillin and other antibiotic drugs were developed, pneumonia was the leading cause of death in the United States. Since 1937, when pneumonia killed more than 110,000 Americans, the death rate from pneumonia has been cut by about two thirds. But much of that decline came in the 1940s and 1950s, with the advent of antibiotics.

The death rate from pneumonia has risen in the past decade, partly because of the AIDS epidemic, worsening air pollution and growth of antibiotic-resistant bacteria. It remains the most common lethal infectious disease.

"The risk of dying from pneumonia, once you have it, has not gone down in 30 years -- despite advances in ICUs, ventilation and antimicrobial therapy," said Joseph H. Bates, chief of the medical service at the Veterans Affairs Hospital in Little Rock, Ark. One reason is that the mortality rate remains high during the first five days after a severely ill patient is hospitalized. Even though bacterial pneumonia is usually treatable with penicillin, the antibiotic may not take hold for several days -- which in the case of a high-risk patient may be too late.

Consider two patients: one admitted to the hospital after a heart attack, and the other admitted with pneumonia. The pneumonia patient's risk of dying "may be much greater," Bates said. "In my hospital, it's more than double."

Patients admitted to the VA Hospital in Little Rock with community-acquired pneumonia had a mortality rate of 28 percent in 1986, he said. For heart attack patients, the mortality rate was 10 percent.

Hospital-acquired pneumonia generally has an even higher mortality rate -- reaching 40 percent in some areas. It occurs in patients who come to the hospital for some other illness and then pick up a pneumonia germ from the ubiquitous "microbial flora" of any hospital. The death rate is higher because those strains tend to be more severe and because hospitalized patients are usually sicker to begin with.

Antibiotic 'Cocktail'

Although Donna Jones would hardly describe her illness as "walking pneumonia," she did not have to be hospitalized and fought it off at home with rest, fluids and antibiotics. Once the diagnosis was made and her antibiotics were stepped up, Jones began to feel better quickly. She recovered much of her energy and was back at work two weeks ago -- although she still tires faster than normally and has a lingering raspy cough.

Jones's was an almost classic case of the most common type of "community acquired" bacterial pneumonia, known as pneumococcal pneumonia. Her doctors could tell by the pattern of clues: her sudden symptoms, the "cloud" in the lower lobe of her right lung, the sputum she coughed up and the way she responded to antibiotics.

But something else about Jones's pneumonia also made it typical. While doctors know it was bacterial, they don't know exactly which of the more than 80 microbial "bugs" implicated in pneumococcal pneumonia was the culprit.

In as many as half the cases of pneumonia, doctors never find out the exact cause -- and therefore treat the patient with a drug that covers a spectrum of possible targets.

"If possible, what should be done is to get a good sputum specimen and look at it under the microscope for clues about what antibiotic 'cocktail' to use," said Henry Yeager Jr., associate professor of medicine in the division of pulmonary and critical care medicine at Georgetown University Medical Center. "But in practice, many doctors go ahead and treat with a broad-spectrum antibiotic, to save time, money and effort."

Less classic cases -- sometimes known as "atypical pneumonia" -- are harder to diagnose. They run a gamut of airborne "bugs," from viruses to mycoplasma to the legionella bacterium to inhaled chemical irritants and dust.

Treatment depends on the type of infection. Bacterial pneumonia is treated with antibiotics -- typically, penicillin or, in patients who are allergic to penicillin, erythromycin. Certain types are treated with other antibiotics, such as ampicillin or cephalosporin.

For viral pneumonia, there is no effective antibiotic and treatment is largely a matter of waiting for the infection to run its course. Many patients with viral pneumonia are still given antibiotics to help prevent secondary bacterial infections.

Pneumonia caused by mycoplasma -- more common in children and young adults -- is usually treated with erythromycin or tetracycline.

Most patients, like Donna Jones, can be treated at home -- hence the term "walking pneumonia." But patients who require intravenous antibiotics or fluid replacement or extra oxygen to help their labored breathing -- and those who have other illnesses that complicate the pneumonia -- are admitted to a hospital.

Even patients who recover quickly and feel fine, said Georgetown's Yeager, should get a follow-up X-ray within four to six weeks "to be sure it has cleared up."

Human lungs contain millions of tiny air sacs that help the body exchange carbon dioxide waste for fresh oxygen to nourish the blood. When a breath is inhaled, air passes through the trachea, or windpipe, and enters the lungs via the bronchi, air passageways that branch out into smaller and smaller passages to the air sacs. Pneumonia is an infection of lung tissue, that causes air sacs to fill up with fluid, pus and inflammation. If too many air sacs are disabled, breathing becomes labored. In severe, untreated cases, the patient dies of oxygen starvation.

No group is immune from pneumonia; it can attack anyone, from infants to the very old. But certain people are at extra risk, meaning that they are more likely to get a lung infection -- or once infected, more likely to become seriously ill or die from it. These include:

Older people. The death rate from pneumonia doubles in patients over 60.

The very young, children under 2, because their airways are still developing.

Smokers.

Anyone with a chronic illness such as heart disease, kidney disease, diabetes or sickle cell anemia.

People with other lung disease, such as emphysema, cancer or chronic obstructive pulmonary disease.

Alcoholics.

People with AIDS.

Organ transplant recipients who have taken immunosuppressive drugs to prevent rejection.

Anyone with leukemia or Hodgkin's disease or who has had the spleen removed.

How do you tell if you might have pneumonia?

The symptoms vary -- and overlap with influenza and other illnesses -- so a chest X-ray, blood tests and laboratory analysis of coughed-up sputum may be necessary to clinch the diagnosis. But here are some telltale signs:

Shortness of breath.

Sharp chest pain when you breathe.

Fever.

Sudden chills, sometimes called "the rigors."

Deep, persistent, noisy cough, often producing greenish-yellow sputum, sometimes flecked with blood.

Blueness around the lips, from lack of oxygen.

Extreme fatigue and lethargy.

Medical experts urge people with those symptoms to see a doctor promptly.