Hillary Clinton is recovering at home after Sunday's health scare revealed that she is being treated for pneumonia.
Clinton's doctor says she "was put on antibiotics, and advised to rest and modify her schedule" before attending Sunday's 9/11 remembrance, where her doctor says she became "overheated and dehydrated."
Clinton left the ceremony early, with video later surfacing on Twitter showing the candidate nearly collapsing as staffers helped her into her van.
Her stumble was caused by pneumonia and dehydration, said Dr. Lisa Bardack, who examined her at her home in Chappaqua.
"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."
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.
Symptoms 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.
Fifteen months ago, a medical report detailed her most serious health issue: Her doctor wrote that in December of 2012, "Mrs. Clinton suffered a stomach virus after traveling, became dehydrated, fainted and sustained a concussion"
She was hospitalized. Doctors found a blood clot, requiring blood thinning medication, which she still takes.
NBC News senior writer Maggie Fox contributed to this report