Hillary Clinton collapsed at a 9/11 ceremony midday Sunday, prompting the usual wild speculation about her health status.

When her campaign finally announced that she had actually been diagnosed with pneumonia on Friday, the pneumonia punditry began. Non-medical professionals of all stripes weighed in on how she should proceed and why she took so long to disclose the serious illness, and suggested medical follow-up.

Never gotten more angrier responses from pro-Clinton tweeters ever than when I said she should have disclosed having pneumonia on Friday. — andrew kaczynski (@BuzzFeedAndrew) September 11, 2016

Friday, day Clinton diagnosed w pneumonia, she appeared at 2 fundraisers, ran a 2-hour natl security mtg, did a presser, sat for CNN intvu — Jennifer Epstein (@jeneps) September 11, 2016

I don't see why the Clinton campaign doesn't have her do another health exam (after he's over the pneumonia) and release. Why not do it? — Jeet Heer (@HeerJeet) September 11, 2016

One thing a lot of the pundits seemed to miss is that people with pneumonia can experience a wide range of symptoms, from the very mild to the deadly. It’s not even one disease: Pneumonia refers to an infection in one or both lungs, and it can be caused by a variety of organisms — bacteria, fungi, viruses, even parasites. (There are some 30 different causes of pneumonia, but the most common cause is the flu.)

The infection essentially leads to inflammation in the lungs’ air sacs, which can fill up with fluid or pus. Symptoms can include a cough, fever, fatigue, chills, loss of appetite, headache, and shortness of breath.

But the severity of pneumonia depends on many things, including the patient’s age and underlying health status. Infants and older people are most at risk of serious infection, as are those with weakened immune systems or other health complications like heart failure or COPD, and smokers.

The source of the pneumonia also affects a person’s prognosis. As the American Lung Association explains:

In bacterial pneumonia, your temperature may rise as high as 105 degrees F. This pneumonia causes profuse sweating, and rapidly increased breathing and pulse rate. Lips and nailbeds may have a bluish color due to lack of oxygen in the blood. A patient's mental state may be confused or delirious. The initial symptoms of viral pneumonia are the same as influenza symptoms: fever, a dry cough, headache, muscle pain, and weakness. Within 12 to 36 hours, there is increasing breathlessness; the cough becomes worse and produces a small amount of mucus. There is a high fever and there may be blueness of the lips.

For children and adults who are otherwise healthy, pneumonia will typically clear up in a few days after a dose of oral antibiotics and some rest. (This kind of pneumonia is usually referred to as "walking pneumonia" because patients aren’t sick enough to be in the hospital.) For those with more severe infections and complications, it can take weeks to get better, and pneumonia can even be deadly.

"[Pneumonia] kills people sometimes," said Lionel Mandell, a pneumonia researcher at McMaster University. "Most of the time it doesn’t — especially if you’re well enough to be treated outside of the hospital."

The campaign hasn’t disclosed the cause of Clinton's particular infection or her symptoms. (The candidate is 68 years old.) And we don’t have a full picture of her health. So let’s stop with the pneumonia punditry.

We know surprisingly little about the true health of presidential candidates

It’s also silly to speculate about Hillary Clinton’s medical status, because her entire health history is not publicly available. And despite the criticism she's received about that, it's unlikely to ever be made public.

As Stat News explained, "Clinton, 68, has so far released only a physician’s letter with about the same amount of detail as letters issued by Barack Obama and Mitt Romney in 2008 and 2012." Her doctor deemed her "in excellent physical condition."

But these letters are essentially meaningless, said Dr. Jacob Appel, an assistant professor at Mount Sinai School of Medicine who has studied the medical histories of candidates on the campaign trial. "No campaign is likely to release a letter stating its candidate is significantly ill or impaired."

He went on: "John McCain in 2008 released summaries of his medical records and gave reporters access to some of his actual records. Other candidates have merely released physicians' testimonials or letters describing a candidate's health in varying degree of detail."

People's medical histories — the diseases they have, past surgeries or infections, their genetic predispositions, their environmental exposures throughout life, the medications they use and how those medications interact — are incredibly complex, and become even more so as we age. Putting all this together is a daunting puzzle for even the most pedigreed clinicians, much less Twitter pundits.

Looking back, we now know a number of past presidents and presidential candidates who were actually much sicker than the public knew. "Most notably, Paul Tsongas and his physicians in 1992 created the impression that he had been cured of non-Hodgkin's lymphoma, which was not the case; he ultimately died of the illness five years later," Appel said. "Well-known are the efforts made to keep FDR's paralysis from public view."

Appel concluded that hiding a candidate's medical history is pretty much the norm: "As someone who studies the health of presidents extensively, I'm convinced that we won't know if either of these candidates is in ill health until history renders its verdict years from now." Let’s keep that in mind as the campaign rolls forward.