If Hillary Clinton brushed aside medical advice to rest after getting a diagnosis of mild pneumonia, she was risking developing a more serious case, medical experts said Monday.

Pneumonia — which leads to infiltration of fluid into the lungs, leaving a patient short of breath and often feverish but still able to function — can become serious or even fatal if it is not properly treated, doctors said.

The illness can be caused by viruses, bacteria or, less often, fungi or damage from toxic fumes. Without extensive testing, which is not normally needed, it is impossible to know what caused Mrs. Clinton’s case.

Mrs. Clinton’s doctor released a statement saying that the illness was diagnosed on Friday morning and that she was advised to “rest and modify her schedule.” Her team has released very little information about her condition: exactly how it was diagnosed; what antibiotics she is taking; the results of any blood work, chest X-ray or other diagnostic tests that may have been performed; or whether she has any underlying condition that made her vulnerable to the illness. On Monday, a campaign spokesman said that more medical information would be released this week and that those records would show she had “no other undisclosed condition.”