Former Vice President Joe Biden appeared to have a blood vessel burst in his left eye while participating in CNN's town hall on climate change.

A broken blood vessel in the eye, also known as a subconjuctival hemorrhage, can be caused by several things, including high blood pressure, bleeding disorders, blood thinners, or even excessive straining.

Biden, 76, has long been plagued by health issues. In 1988, he suffered an aneurysm that burst and required him to undergo emergency surgery. The then-senator was so close to death that a Catholic priest began preparing to administer the sacrament of last rites.

Months later, surgeons clipped a second aneurysm before it burst. Biden then took a seven-month leave from the Senate following the surgery. Describing the operation, he once said, “They literally had to take the top of my head off.”

Jill Biden said in her recently released autobiography Where the Light Enters that, at the time, she feared her husband would never be the same. "Our doctor told us there was a 50-50 chance Joe wouldn't survive surgery," she wrote. "He also said that it was even more likely that Joe would have permanent brain damage if he survived. And if any part of his brain would be adversely affected, it would be the area that governed speech."

Doctors removed a benign polyp during a colonoscopy in 1996. In 2003, Biden had his gallbladder removed.

He suffers from asthma and allergies and takes a prescription drug to lower his cholesterol. He has also taken medication for an enlarged prostate.

Biden hasn’t disclosed his medical history since 2008, when doctors found he had an irregular heartbeat.

Biden has also raised eyebrows for the increasing number of verbal blunders he has made so far on the 2020 campaign trail, the schedule of which has been markedly lighter than his main rivals.

Those close to Biden nevertheless maintain that he is "a picture of health," according to a former aide who spoke to the Washington Examiner in April. Were he to win the 2020 presidential election, he would be the oldest president ever to be inaugurated.