Joe Biden probably needs another COVID-19 test after sharing a debate stage with Trump — but if he gets sick, his weight and fitness are on his side

• Joe Biden tested negative for COVID-19 on Friday after President Trump tested positive.
• The former vice president should be re-tested because it's been only two days since he shared a debate stage with President Trump, where he could have been infected.
• If he were to test positive, his age (77) and sex make him vulnerable to severe illness and even death.
• But his healthy habits, like not smoking and exercising regularly, are protective.

Former Vice President Joe Biden tested negative for COVID-19 today, hours after President Trump revealed a positive test.

Biden, the Democratic nominee, still could have contracted the novel coronavirus while sharing a debate stage with Trump Tuesday.

Emerging evidence suggests people are most contagious two days before they experience symptoms. Trump was reported to start feeling symptoms on Wednesday. As such, he would have been contagious Tuesday night while on stage with Biden.

Despite his negative result, it typically takes four to five days between exposure and symptom onset, so Biden's test may have been too soon. In fact, the Minneapolis Department of Health recommends people get tested five to seven days after an event where they might have been exposed.

If Biden does test positive later, at 77 years old, he's at higher risk for serious illness from the disease. But other factors, like his commitment to exercise, are protective.

The virus thrives in indoor settings with raised voices

The debate's indoor setting also made it amenable to the virus's spread.

While Trump and Biden remained more than six feet apart and didn't shake hands, the fact that the men didn't wear masks and frequently raised their voices isn't good news for Biden's chances of a negative test.

For one, the six-foot rule is an arbitrary outdated measurement designed 80 years ago as a guidelines, not a hard-and-fast rule, Insider's Hilary Brueck previously reported.

There is also evidence the novel coronavirus, which causes the disease COVID-19, acts differently in indoor spaces with poor ventilation and/or no open windows.

The virus seems to mostly spread through respiratory droplets, which generally drop to the ground within six feet. However, the World Health Organization said it's possible that, when indoors, the coronavirus can be transmitted through aerosols — smaller, lighter particles which can linger in the air over several feet.

What's more, research suggests singing and loud talking can propel viral particles farther than six feet, and some scientists even advocate avoiding talking, especially indoors.

"There's no safe distance in indoor spaces," Dr. Ali Nouri, president of the Federation of American Scientists, tweeted on Friday.

Biden's age and sex make him susceptible to severe illness

Biden's age makes him particularly vulnerable to getting a severe case of COVID, if he does test positive.

According to the Centers for Disease Control and Prevention, people who are 75- to 84-years-old are eight times more likely to be hospitalized and and 220 times more likely to die from the disease than 18- to 29-year-olds.

His sex puts him in a higher-risk population too. Global data has suggested that men are more likely than women to get a deadly infection, and the CDC shows that men have made up 54% of all deaths in the US.

Biden's weight and healthy habits are on his side

On Tuesday, Dr. Kevin O'Connor of The George Washington University released a three-page medical summary of Biden's health describing the candidate as "healthy, vigorous, 77-year-old male, who is fit to successfully execute the duties of the Presidency to include those as Chief Executive, Head of State and Commander in Chief."

While Biden hasn't had a perfect health history — he's had two brain aneurysms, one that was complicated by deep vein thrombosis and a pulmonary embolism — O'Connor said Biden doesn't smoke or drink alcohol, and is committed to exercising at least five days a week.

Those habits boost his chances of both avoiding the virus altogether and fighting it successfully if he does get ill.

People who smoke are more likely to experience serious illness and death from COVID-19, and regular exercise may help Biden maintain a healthy weight, another factor linked with less serious disease. Obesity is connected with a higher rate of hospitalization and death from the illness.

Regular exercise is also beneficial for the immune system, and one study even suggested it supports the production of an antioxidant that protects against acute lung disease and other diseases.

In particular, the antioxidant can protect against acute respiratory distress syndrome, or ARDS, which is deadly in 45% of cases. ARDS affects up to 85% of patients in the ICU affected by COVID-19.

"Regular exercise has far more health benefits than we know," study author Zhen Yan said in a press release. The protection against this severe respiratory disease condition is just one of the many examples."