This isn't the first time Clinton has suffered a blood clot. In 1998, midway through her husband's second term as president, Clinton was in New York fundraising for the midterm elections when a swollen right foot led her doctor to diagnose a clot in her knee requiring immediate treatment.
Clinton had planned to step down as secretary of state at the beginning of President Obama's second term. Whether she will return to work before she resigns remains a question.
Democrats are privately if not publicly speculating: How might her illness affect a decision about running for president in 2016?
After decades in politics, Clinton says she plans to spend the next year resting. She has long insisted she had no intention of mounting a second campaign for the White House four years from now. But the door is not entirely closed, and she would almost certainly emerge as the Democrat to beat if she decided to give in to calls by Democratic fans and run again.
Her age -- and thereby health -- would probably be a factor under consideration, given that Clinton would be 69 when sworn in if she were elected in 2016. That might become even more of an issue in the early jockeying for 2016 if what started as a bad stomach bug becomes a prolonged, public bout with more serious infirmity.
Not that Democrats are willing to talk openly about the political implications of a long illness, choosing to keep any discussions about her condition behind closed doors. Publicly, Democrats reject the notion that a blood clot could hinder her political prospects.
"Some of those concerns could be borderline sexist," said Basil Smikle, a Democratic strategist who worked for Clinton when she was a senator. "Dick Cheney had significant heart problems when he was vice president, and people joked about it. He took the time he needed to get better, and it wasn't a problem."
It isn't uncommon for presidential candidates' health -- and age -- to be an issue. Both in 2000 and 2008, Sen. John McCain, R-Ariz., had to rebut concerns he was too old to be commander in chief or that his skin cancer could resurface.
Beyond talk of future politics, Clinton's three-week absence from the State Department has raised eyebrows among some conservative commentators who questioned the seriousness of her ailment after she canceled planned Dec. 20 testimony before Congress on the deadly attack on the U.S. diplomatic mission in Benghazi, Libya.
Clinton had been due to discuss with lawmakers a scathing report she had commissioned on the attack. It found serious failures of leadership and management in two State Department bureaus were to blame for insufficient security at the facility. Clinton took responsibility for the incident before the report was released, but she was not blamed. Four officials cited in the report have either resigned or been reassigned.