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Hillary Clinton's wobbly incident Sunday is a near-textbook case of what can happen with "walking pneumonia" — patients don't feel great, but they're not sick enough to stay home in bed or to be hospitalized.

Clinton caused daylong speculation when she left a Sept. 11 memorial service in New York on Sunday morning and was seen being helped into an SUV, none too steady on her feet.

It turns out it was pneumonia and dehydration, said Dr. Lisa Bardack, who examined her at her home in Chappaqua.

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She's had it for a while.

"Secretary Clinton has been experiencing a cough related to allergies. On Friday, during follow up evaluation of her prolonged cough, she was diagnosed with pneumonia," Bardack said in a statement.

"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."

Nick Merrill, a spokesman for Clinton, said Clinton would remain at her home in Chappaqua, N.Y., to rest. A planned trip to California for events Monday and Tuesday was canceled, he said.

Pneumonia can be caused by viruses or bacteria, and it refers to an infection that gets into the lungs. Bacterial pneumonia is common, usually not serious and easily treated with antibiotics.

These mild bacterial infections can cause a cough and sometimes a mild fever.

People often feel well enough to go about their business, especially once they start taking antibiotics. And once they've been taking antibiotics for a day or so, they are unlikely to infect anyone else.

But the infection can take a toll, making patients prone to getting tired and dehydrated. And since people often don't realize they have an infection, they can go for days or weeks before they seek treatment — all time for the infection to wear them out even more.

Related: Is Pneumonia Contagious?

There's a vaccine that's designed to protect people over 65 from pneumonia-causing streptococcal bacteria, but many other bacteria, from Staphylococcus to Mycoplasma pneumoniae, can cause pneumonia.