When Democratic Party nominee Hillary Clinton appeared on a late-night talk show last week, host Jimmy Kimmel checked her pulse before making her open a jar of pickles.

It was meant as a light-hearted jab at the questions that are being raised about the health of the former senator - who is approaching her 69th birthday - but it also highlighted the growing chatter about the relative well-being of both candidates.

Amid the ugly to-ing and fro-ing over racism in recent weeks, Mrs Clinton's and Republican candidate Donald Trump's campaigns have begun casting doubts on the state of health of each other's candidate.

Mr Trump, speaking at a recent rally, said Mrs Clinton did not have the "mental and physical stamina" to confront the Islamic State in Iraq and Syria.

Mr Trump's surrogate, former New York mayor Rudy Giuliani, urged people to look for online videos that show she has an illness.

Mrs Clinton's campaign, meanwhile, posted a tweet on Monday that raised questions about a letter from Dr Harold Bornstein that, thus far, constitutes the only medical record Mr Trump has released.

The tweet included a link to a 13-point critique of the letter, which took issue with everything from a typo in the opening line to the doctor's conclusion that Mr Trump, 70, would be the "healthiest individual ever elected to the presidency".

"What a whopper of a closing statement! We feel confident saying Dr Bornstein has never examined George Washington," said the online critique. As with so many other things in this year's election, the issue of health has frequently taken a turn for the bizarre.

The author of that four-paragraph letter recently admitted in an interview with broadcaster NBC that he wrote it in five minutes while a limousine - sent by Mr Trump - waited outside his office. Dr Bornstein, however, stood by his assessment. "His health is excellent, especially his mental health," he said.

Away from the official campaigns, attacks on the health of the two candidates are more peculiar. Right-wing blogs have recently circulated photos of Mrs Clinton propped up with pillows and a video edited to make it appear as if she has had a seizure.

Reporters who were present at the time that the video was taken said the jerky movement was a product of Mrs Clinton acting surprised - not having a health episode.

Meanwhile, some therapists have started diagnosing Mr Trump from afar, saying he appears to be a textbook case of someone with narcissistic personality disorder.

Though both candidates dismiss any concerns about their health as distractions, they have also disclosed very little.

Mrs Clinton has released a medical report and Mr Trump released that letter. In contrast, during the 2008 election, Republican candidate John McCain, who was then 71, let journalists have access to more than 1,000 pages of his medical history.

Analysts generally agree that the health records of presidential candidates are relevant topics of discussion in any given election year - all the more so when the two candidates of the major parties are among the oldest in history.

If Mr Trump wins the White House, he will be the oldest first-term president. If Mrs Clinton wins, she will be the second-oldest after the late Ronald Reagan, who started his first term in 1981, a few weeks shy of his 70th birthday.

Mrs Clinton had a health scare in 2012 when she fell and had a concussion that took months of recovery.

Dr Barbara Perry, director of Presidential Studies at the University of Virginia's Miller Centre, said there has been a long history of presidential candidates being less than upfront about their health - from Franklin Roosevelt's polio to John F. Kennedy's Addison's disease.

However, there is currently still no requirement for presidential candidates to disclose their medical records or to undergo a medical check-up.

Dr Perry is among those who have started calling for rules of disclosure, noting that for many other occupations, employers consider the health of their candidates before hiring. But she added that it can be a complicated issue.

"We need to have some kind of rules for disclosure, but we also do want to be careful not to make it so that all diseases become disqualifying. So many of our presidents have served with health problems." She added that it was not clear if the current debate about the candidates' health would affect the race.

She said: "It's such an important position and we can't take the risk of having a president in ill health. It's surprising how often the topic will come up but not become a major issue because, I think, we Americans don't deal properly with the issue of a candidate's health."