During a coughing fit at a rally in Cleveland on Sept. 5, Democratic presidential nominee Hillary Clinton joked she was allergic to thinking about her Republican rival Donald Trump. (Gillian Brockell/The Washington Post)

On Labor Day, in the middle of a Cleveland campaign rally, Democratic presidential candidate Hillary Clinton paused to fight back a cough. She sipped from a glass of water, turned from her microphone and then shrugged the spell off. “Every time I think about Trump,” Clinton said, “I get allergic.”

If only stump speech quips could defeat microbes. Six days after the Ohio rally, traveling from a Sept. 11 memorial service held in New York, Clinton lost her balance and had to be helped into a waiting van. Passersby caught the stumble on video and forced the campaign to come clean: Clinton had fallen ill. A few days before, it turned out, a physician diagnosed the 68-year-old candidate with pneumonia.

“Secretary Clinton has been experiencing a cough related to allergies,” Clinton’s physician Lisa R. Bardack said in a statement, as The Washington Post reported Sunday. “On Friday, during follow up evaluation of her prolonged cough, she was diagnosed with pneumonia. She was put on antibiotics, and advised to rest and modify her schedule. While at this morning’s event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely.” Clinton canceled a planned trip to California.

Video of Clinton's departure seemed to show her buckling and stumbling as she got into her van. (Thomas Johnson/The Washington Post)

Although Bardack said her patient was on the way to recovery, the diagnosis of pneumonia may have left some voters wondering: Isn’t pneumonia, after all, a serious health concern?

The short answer, at the moment — based on the limited available information about Clinton’s case and viewed independently of any unknown other health issues she may or may not face  — is no.

“At this point, there is no reason to believe that Secretary Clinton will be disabled” by pneumonia, American Lung Association scientific adviser Norman H. Edelman told The Post by phone Sunday night.

The long answer to pneumonia’s seriousness begins more than a hundred years ago, when pneumococcal bacteria were discovered in the 1880s. Near the dawn of the previous century, that disease had replaced consumption, or what we would now call tuberculosis, as the primary cause of death in cities such as Chicago, according to Canadian physician William Osler. He made a scrupulous study of the disease and described it in almost poetic terms. In 1901, he wrote, “The most widespread and fatal of all acute infectious diseases, pneumonia, is now the ‘Captain of the Men of Death.’”

Osler also observed that pneumonia frequently struck those whose health was already compromised. He gave the illness another name, “the old man’s friend,” as it was a terminal — though by standards of the time, painless — affliction among the elderly. As it were, Osler would get to befriend the disease personally. He succumbed to pneumonia in 1919.

Had he lived for two more decades, the doctor would have seen penicillin change the world. After World War II, the development of the drug and other antibiotics slashed the disease’s mortality rate to nearly a third, compared with the early 1900s. Pneumonia fell from its perch at the top of lethal causes, though not very far. Together with influenza, the disease was still the eighth-leading cause of death in the United States in 2013. That year, it had a mortality rate of 17 deaths per 100,000 people, according to the Centers for Disease Control and Prevention.

Death remains a part of the pneumonia spectrum. Not all pneumonias, though, are created equally.

“The problem is we have one word for a disease that comes in lots of different iterations,” Edelman said.

You have probably heard of pneumonia, but may not understand what it actually is. This video explains the science behind the condition. (Courtesy of Osmosis (www.osmosis.org[osmosis.org]), and available under a CC BY SA license.)

At its most basic, pneumonia simply means that a germ has inflamed the lung’s air sacs. Cold viruses infect the nose and throat; bronchitis concerns the airways; and pneumonias afflict the dark, wet and warm parts of the lung itself. Making matters more complicated, the pathogens chewing into the lung tissue could be any one of a motley crew of bacteria, viruses or fungi.

The severity of pneumonia is as broad as its possible culprits. At the far end lies a mild disease. The illness is known colloquially as walking pneumonia, so named because the disease rarely knocks anyone out of commission and into bed. Here is likely where we find Clinton, at least based on what her campaign has revealed so far.

The CDC estimates that 2 million people a year fall sick with such milder forms of pneumonia. “Developing walking pneumonia does not mean you have poor health,” Stanford University pulmonologist Mark Nicolls said in an interview with The Post. “Young and healthy people develop walking pneumonia.” It is possible there are even more cases than that CDC estimates, too, as walking pneumonia is difficult to diagnose. Symptoms rarely stray from coughing, fever, fatigue and other average maladies.

If Clinton indeed has walking pneumonia, there is a decent chance — odds range between 1 to 50 and 1 to 5 — that the culprit is a bug named Mycoplasma pneumoniae. Among bacteria, M. pneumoniae is an oddity. It is both exceptionally small — one of the smallest organisms on Earth — and unusually vulnerable, as it lacks the cell walls that sheath most other bacteria. As far as is known, it is an exclusive human parasite, unable to reproduce outside warm and wet human organs. M. pneumoniae is spread person to person, through droplets expelled through the respiratory tract.

But that is just one possibility. “The bugs that cause pneumonia are common,” Edelman said. “Lots of people carry those bugs in their nose and throat.” (This was about as close as an expert would speculate about the risk factor of shaking sweaty hands by the millions on the campaign trail.)

It is too soon to say with certainty if M. pneumoniae or any other microorganism is responsible for Clinton’s pneumonia. The laboratory tests to determine a type of mild pneumonia, in fact, frequently take longer that the disease itself lasts.

Even the fact that Clinton is taking antibiotics offers little perception into the nature of her illness. (Antibiotics are only effective against bacteria, not viruses.) Physicians may administer such drugs in suspected bacterial cases. A chest X-ray, in the meantime, will give at least some insight. Such an image can “show a diffuse whiteness that can look worse than the patient does,” Nicolls said. Viruses, generally speaking, are inclined to spread out, appearing on a lung X-ray in a smothering white fog. Other sorts of microbes stay close. “The bacteria tend to get localized to a lobe of the lung,” he said.

No matter the specific bug, this sort of pneumonia is what’s known as community-acquired pneumonia. That is, it was caught anywhere but in a health-care system. Hospital-acquired or health-care-associated pneumonias are much more severe diseases, often involving drug-resistant pathogens. Those diseases are a large part of why the umbrella of pneumonia continues to have a high mortality rate.

What is far less likely, too, is that Clinton caught the pneumococcal form of the disease. The CDC recommends anyone older than 65 to be vaccinated for the more severe pneumococcal type. At 68, Clinton falls within this demographic. (The American Lung Association’s advice is also to vaccinate at 65.)

Walking pneumonia is ultimately self-limiting, which means the disease should pass on its own, even without antibiotics. “It’s common. It occurs at all ages,” Edelman said. “It occurs in perfectly healthy people as well as those who are sick.” 

In fact, on Friday, the day of her diagnosis, Clinton attended several events, including a CNN interview and a national-security meeting. “If she’s had it for more than a week or so,” Nicolls said, “you could say that it indicates a certain amount of stoicism.”

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