A wide variety of medical information can be important in judging the length of a recovery period, which varies among types and individuals. Some have “walking pneumonia,” which is a nonspecific phrase, and still go to work. Depending in part on the kind of pneumonia, others may need days to weeks of rest.

History of Blood Clots

Mrs. Clinton has experienced blood clots on three occasions, two of which developed in a leg vein: one in 1998 when she was first lady, and another in 2009 when she was secretary of state.

The first occurred during midterm congressional elections when she developed a swollen calf from phlebitis and a blood clot. Mrs. Clinton’s doctors advised that she be hospitalized, but she refused, citing the demands of the campaign. (Mrs. Clinton similarly declined to cancel events after being told she had pneumonia.) She received outpatient treatment without known complications. Mrs. Clinton has not released her medical records from her White House years.

In 2012, Mrs. Clinton experienced a stomach virus after foreign travel. She became dehydrated, fainted and fell at her home in Washington, striking her head in the process. She experienced a concussion and double vision, for which she wore Fresnel Prism glasses for a period. A follow-up magnetic resonance imaging scan at the time showed a clot in one of the two main veins that drain blood from the brain. Doctors prescribed Coumadin, an anticoagulant drug, or “blood thinner,” to prevent the clot from growing and becoming even more dangerous.

Medications

Other tests showed that Mrs. Clinton did not have an underlying disorder that increased her risk for developing blood clots, Dr. Bardack said. Nevertheless, as a precaution, she continues to take Coumadin (also known as warfarin) daily to prevent further clots. A number of dietary items and other factors can strongly influence Coumadin’s effects on the blood-clotting system. These are among the reasons such patients frequently have a blood test, known as an I.N.R., to monitor the dose.

Many patients have switched to newer drugs like Pradaxa, Xarelto and Eliquis that eliminate the need for blood tests and allow for easier clotting control. However, many doctors advise patients who have stable I.N.R. test results and have not developed complications to stay with Coumadin. Dr. Bardack said she and Mrs. Clinton’s hematologist had decided not to switch to a newer anticoagulant.

Mrs. Clinton takes no additional regular medication except for thyroid hormone, which tests show properly controls her underactive gland; Clarinex for allergies; and vitamin B-12 when she feels she needs it.