Dr. Bill Fisher blasted off Earth in 1985 aboard a NASA space shuttle, speeding 18,000 mph through the atmosphere.

But Dr. Fisher, now a 74-year-old emergency room physician, tweeted Wednesday: "I am genuinely more concerned about going to work tomorrow morning than I was the day I launched on the space shuttle."

That's because the coronavirus (which causes the respiratory disease COVID-19) is easily spread, can be excruciating, and is most deadly in people over 60. (Though the pathogen can seriously sicken young people, too).

Seven crewmembers tragically died when the space shuttle Challenger broke apart after launching in 1986. In 2003, seven more shuttle astronauts perished when the Columbia disintegrated upon returning to Earth. In total, Dr. Fisher wrote that he had a 1.5 percent chance of dying on a space shuttle, which flew 135 missions over 30 years.

Any way you cut it, someone in Dr. Fisher's age group has a much higher probability of dying from a coronavirus infection, which is currently the deadliest infectious disease in the world, overtaking tuberculosis.

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Getting infected at all is often serious. About one in five infected people need to be hospitalized, according to the World Health Organization.

Overall, COVID-19 is certainly deadlier than the flu. Each year, the flu kills around 0.1 percent of those infected. Meanwhile, a new study released on March 30 in the journal The Lancet Infectious Diseases found that the overall fatality rate from COVID-19 is 0.66 percent — making it over six times more lethal than the flu. Importantly, we're still in the relatively early stages of the pandemic, so this number will almost certainly change and grow increasingly certain as more data comes in. Previously, Anthony Fauci, the well-regarded director of the National Institute of Allergy and Infectious Diseases, estimated that overall COVID-19 is 10times more lethalthan the flu.

But as Dr. Fisher noted, he's in an older age group.

A recent CDC report found the coronavirus fatality rate is between 3 percent to 11 percent in Americans aged 65 to 84. Three percent is 30 times more deadly than the flu. (Flu deaths in the U.S. are dominated by the elderly, with 70 to 85 percent of deaths occurring in people 65 and older).

"The death rate for people ages 70-79 in Italy has been 12.8%," Dr. Fisher tweeted, explaining the mortality numbers cited in his first tweet. "In China it was listed at 8%. Currently in the US we are somewhere between 4-5%. Of course, nobody really knows. Whatever it is, It’s not a trivial number."

The pronounced mortality numbers for older people (Americans aged 20–54 years have a mortality of less than 1 percent) is a critical reason why infectious disease experts implore younger folks to social distance. If they don't, they endanger the lives of older people, like Dr. Fisher.

"In order for social distancing to work, everyone has got to do it," Dr. Timothy Murphy, an infectious disease researcher at the University of Buffalo, told Mashable. "It’s not going to work unless everyone buys into it." 

But doctors like Fisher are inherently put in high-risk environments, as they're surrounded by COVID-19 infections.

"Some of your comments question the wisdom of what I am doing," Dr. Fisher tweeted in response to questions about why he was risking his life. "I have been practicing emergency medicine for 45 years. I like it, it’s interesting, and I have lots of experience. To abandon ship during the greatest public health crisis of our lifetimes is inconceivable to me."

"Our medical professionals are heroes putting their lives on the line to keep our country going," former president Barack Obama tweeted in March.