I was diagnosed with pneumonia in October. The doctor told me to rest, really rest. She told me to expect to feel better after a couple of days of antibiotics, but that I still must rest. She told me I would have good days, but they would be followed by bad days.

After a week of antibiotics, the bacteria causing my illness – presumably Streptococcus pneumonia – should have been dead. Also called pneumococcus, this pathogen is the most common perpetrator of community-acquired pneumonia, which is pneumonia that people get outside hospitals and nursing homes. The antibiotic I received, a common first-line treatment, covers pneumococcus as well as other bacterial invaders.

Yet my doctor told me to expect weeks to months of recovery. Friends with recent pneumonia experience confirmed this rather depressing outlook. Pneumonia can vary in severity so not everyone will need months to recover. (Hillary Clinton, who had a highly publicized case of pneumonia in September, was diagnosed with a mild form and only took a few days off from the campaign trail.)

The scientific literature concurs with the anecdotal evidence I collected. One study followed 576 adult patients with community-acquired pneumonia. Thirty days after diagnosis, 65 percent of them reported fatigue, nearly half of whom said their fatigue was moderate to severe; 53 percent reported cough and 36 percent reported shortness of breath. Ninety days after diagnosis, 51 percent reported fatigue, 32 percent cough, and 28 percent shortness of breath.

Another study surveyed 500 pneumonia patients age 50 and older and found that tiredness, weakness and shortness of breath lasted more than three weeks, on average.

In addition to the burden of illness on patients, researchers noted burden on family members-turned-caregivers and on the health-care system, including multiple visits to doctor’s offices if not the emergency room or the hospital, says study co-author John Powers, an internist and infectious-disease specialist at George Washington University.

Pneumonia is a serious and potentially fatal disease. In Powers’ study, about 40 percent of people with community-acquired pneumonia ended up in the hospital. Pneumonia and flu together are on the top 10 list of causes of death, according to the Centers for Disease Control and Prevention.

A 2011 study of health-care records estimated that 866,000 cases of pneumonia occurred in the United States in 2004; in patients age 65 or older, pneumonia caused 242,000 hospitalizations and 16,000 deaths.

I went back to the doctor after two weeks because of intensely uncomfortable shortness of breath. She prescribed oral steroids in addition to a rescue inhaler to calm my overactive airways, which helped. By Day 30, I felt reasonably well most of the time, but I still needed daily naps.

Why does it take so long to recover from pneumonia?

One reason is that the detritus from an infection of the lung is hard to clear. Antibiotics kill the bacteria, but all the weaponry your body produced to fight the bacteria – mucus, essentially, or sputum, as it’s called once you cough it up – is left behind.

“Your clearance mechanisms have to take all that stuff out,” says Steven Simpson, acting director of the division of pulmonary disease and critical care medicine at the University of Kansas. Your airways are lined with hairlike cilia that consolidate microbes and mucus and help move it toward the exit.

“It literally takes a lot of energy to keep yourself going with all that stuff in your lungs,” Simpson says.

Cough is a primary way to clear the gunk. That’s why doctors advise pneumonia patients not to take cough suppressants. You want to get that stuff out.

It’s harder to explain the lingering of symptoms such as fatigue and weakness. “We really don’t understand the biology of this,” says Norman Edelman, senior scientific adviser for the American Lung Association, who practices medicine at the State University of New York at Stony Brook.

“Most people think that illness is related to the organism. That’s only one part of the story,” Powers says. “Some symptoms, such as cough and chest pain, clear up relatively quickly. Fatigue, however, takes much longer. That’s because your immune system is still revved up.”

The infection sets off a cascade of events that ramps up inflammatory and immune response, Powers says. “You have the bug, and you have the host response to the bug.”

A revved-up immune response requires a lot energy. “Your body goes into a mode where it’s diverting energy to the immune system,” Powers says.

Simpson says the energy drain burns calories and proteins. When illness dampens appetite, that can exacerbate fatigue and weakness. He advises: “Eat good protein and take plenty of calories.”

And don’t forget to rest – really rest. “There’s no reason to confine yourself to your bed, but don’t push it,” Edelman says. “Don’t make yourself exhausted.”

What about shortness of breath? This was the most bothersome of my lingering symptoms. Air would suddenly feel thick in my lungs, and my upper back would start to ache – the stress of labored breathing, my doctor said.

“Pneumonia can trigger a syndrome that is asthmalike,” says Powers, who has experienced it. “It’s not asthma. It’s a hyper-responsiveness of the airways.” Triggers such as exercise or cold air can lead to that very tight feeling in the chest and labored breathing.

I felt as if I had a good idea of the scope of this illness – both from my doctor and from a handful of friends with experience. My case was not severe by any measure – I was never considered for hospitalization – and I don’t have preexisting conditions affecting my lung function, such as asthma or chronic obstructive pulmonary disease. I can work from home and steal naps during the day. Yet I was still unprepared to deal with the worst bad days and sought medical care three times in that first 30 days.

This, too, is not unusual, Edelman says. “As physicians, our main job is to reassure patients.”

Special to The Washington Post · Jill U. Adams