Democratic front-runner Joe Biden was speaking at a televised town hall Wednesday when the corner of his left eye appeared to fill with blood, bringing attention away from the issue of climate change at a time when his campaign is already batting back scrutiny about his age and health.

The eye issue likely isn't serious and will go away on its own after a few days, but it came after reporters have been asking Biden about whether his age will get in the way of his pursuit of the White House and followed a string of verbal miscues. Biden is 76, and if elected he would be the oldest person ever to occupy the Oval Office, older than Ronald Reagan at the end of his second four-year term.

By the time any second Biden term ended, he would be 86, uncharted territory for the presidency.

"Hopefully, this is no big deal," tweeted Herman Cain, a former Republican presidential candidate, about Biden's eye. "That said, it's not helping Joe Biden — a man who desperately needs to dispel the idea that his age and health are 2020 concerns."

[Also read: With a spring in her step, Warren, 70, breezes past scrutiny of her age]

Hopefully, this is no big deal. That said, it's not helping Joe Biden - a man who desperately needs to dispel the idea that his age and health are 2020 concerns. #2020election #JoeBiden https://t.co/LwcpJtY9jV — Herman Cain (@THEHermanCain) September 5, 2019

Biden's eye condition looks scarier than it is. It is the result of a burst blood vessel. It's common for any age, and can happen to people after they keep contact lenses in too long, lift something heavy, or even after they cough or sneeze. For Biden, it has happened at least once before in public: In 2013, Twitter buzzed about the then-vice president's eye as he sat behind President Barack Obama during his State of the Union address. His eye appeared bloody after he reportedly scratched it on his contact lenses, and he was sporting glasses.

This time was different because it happened spontaneously in plain view. The Biden campaign would only talk about what it said the cause was on an off-the-record basis, raising the question why it did not want to be connected to a simple explanation that would have cleared up the issue.

Biden hasn’t disclosed his medical history since 2008, when doctors found he had an irregular heartbeat. It has already been reported that he battles with asthma and allergies, and, like Trump, takes a prescription drug to lower his cholesterol. Biden also has taken medication for an enlarged prostate and had his gallbladder removed in 2003.

“In general, there is no reason for alarm,” said Dr. Thomas Steinemann, spokesman for the American Academy of Ophthalmology, upon seeing images of Biden from the town hall. “This is basically a bruise on the white part of your eye. It looks terrible but it's truly one of those things that looks bad but for the most part is not serious. It does not hurt, it does not cause you to go blind, and it does not cause changes in vision."

In rarer cases, such pooling of blood is a sign that a patient's blood thinners need to be adjusted. But Steinemann, who also teaches ophthalmology at the MetroHealth System in Cleveland, Ohio, stressed that for eye bleeding, as with much of medicine, the "common causes are the common explanations."

The Biden campaign has not disclosed whether the former vice president takes blood thinners, a common medication for older Americans, and neither he nor any of the other candidates have yet released their medical records. The campaign declined to offer a comment on the record. Trump released little information about his annual physical this year.

Biden's most serious health issues happened more than 30 years ago, when he had an aneurysm that burst and caused him to undergo emergency surgery. Months later, surgeons clipped a second aneurysm before it burst.

If Biden is on blood thinners, it wouldn't be on account of his aneurysms, as the two are not related. Blood thinners help to prevent strokes, while aneurysms happen when the walls of an artery become weak, causing them to bulge and sometimes burst.

More recently, Biden's verbal blunders have drawn headlines. He said poor kids are "just as bright" as white kids, claimed he was vice president in 2018 during the Parkland massacre, mixed up Iowa and Vermont, and referred to Margaret Thatcher, who left office in 1990 and died in 2013, as British prime minister instead of Theresa May.

Trump has accused Biden of "not playing with a full deck," and Biden has lashed back. He has raised the president's comments about the trade war with China, in which Trump called himself the "chosen one" to take on the fight, and said Trump was feeling under pressure after a majority of economists predicted the economy would soon lapse into recession.

"The fact is that day by day we are witnessing a president of the United States who has become more and more unhinged," Biden said.

Some found the media reaction to the bloody eye excessive and offered sympathy. But presidential candidates traditionally prefer to project strength rather than elicit pity.

John Aldrich, political science professor at Duke University, said that when candidates have health issues closer to elections, including the primaries, they may influence how people vote, but otherwise they are considered far in the rear view mirror for voters who have decided on a candidate already.

Voters also can be influenced when something new emerges about a candidate's health, and not just the same issues that come up over months or years, he said. Aldrich otherwise has observed that, in general, candidates' attacks on each others' health tend only to reinforce voters' already-established political proclivities.

"You either agree with Biden or don't agree with Biden about Trump being unhinged," Aldrich said. "No one is going to say, 'Oh, that's what’s been bothering me about Trump!’”

Biden isn't the only Democratic candidate more advanced in age. Bernie Sanders is older than Biden, and if he wins, he would be 79 by the time he reaches the White House. Elizabeth Warren of Massachusetts would be 71.

And health foibles in full public view are hardly new on the campaign trail. In the run-up to the 2016 election, Democratic nominee Hillary Clinton drew widespread scrutiny at a 9/11 memorial event where she stumbled, the result of pneumonia.

When he was running for the Republican nomination for president in 2011, Rick Perry forgot the name of the Energy Department (the agency he now leads under Trump), and later reports emerged saying his campaign struggled for months because he was suffering from post-surgery back pain.

In 2007, Rudy Giuliani's campaign plane was turned back after it took off from Missouri because he was experiencing "a severe headache and flu-like symptoms" and the presidential candidate, then 63, was taken to the hospital overnight. Some former aides believe his lackluster approach to campaigning was in part because he didn't feel well.

When he competed for the Democratic nomination in 1992, Sen. Paul Tsongas of Massachusetts talked about his survival from a form of cancer known as non-Hodgkin's lymphoma, saying it had been cured. But the disease was incurable and Tsongas died at the age of 55 in January 1997. If he had been elected president instead of Bill Clinton, he would have died in office.