"Back in October, the National Enquirer said I would be dead in six months. "So with every breath I take, I feel like I have a new lease on life," Clinton joked to Kimmel. "I do feel sometimes like this campaign has entered into an alternative universe."
Rival Donald Trump has repeatedly argued that Clinton does not have the "stamina" to be president, and Republican critics have attempted to cast doubt on her health - claims her campaign has described as "right-wing conspiracy theories".
Over the weekend, the Republican presidential candidate added fuel to the rumours that she has been "missing in action" of late, giving the globally trending hashtag #WheresHillary a boost with a theory of his own.
During a foreign policy speech last Monday, Trump commented that Clinton "lacks the mental and physical stamina to take on ISIS".
The allegation was quickly denounced by her campaign, which pointed to a doctor's letter released last year that pronounced her health "excellent".
Referring to the Trump campaign's push to paint her as unhealthy, Clinton told Kimmel on Monday night: "I don't know why they are saying this. I think on the one hand it is part of the wacky strategy - just say all these crazy things and maybe you can get some people to believe you.
"On the other hand, it absolutely makes no sense. I don't go around questioning Donald Trump's health. I mean as far as I can tell, he's as healthy as a horse," she said.
The issue of Clinton's physical and mental health was most recently raised by former New York City Mayor Rudy Giuliani, a close adviser to Trump. "I think Hillary's tired ... she looks sick," he told Fox News on Monday, doubling down on his comments over the weekend when he urged people to Google "Hillary Clinton illness" to support his theory.
Clinton's campaign later shot back on Twitter, suggesting people "Google Rudy and health" to read about why he withdrew from the 2000 Senate race, which was directly linked to his health.
