In 1985, a doctor named Bill Fisher took off as an astronaut and part of the crew on the Space Shuttle Discovery.

Now, 35 years later, he's on another mission.

Fisher, aged 74, is an emergency room doctor in a Texas hospital--working 24 hour shifts on the front line of the Covid-19 crisis. With a short statement this week, he became an inspiring example for anyone trying to stay motivated to keep working against difficult odds.

If you're an entrepreneur or a business leader, or anyone trying to steer a ship in difficult times, you probably need to hear it.

'Genuinely more concerned'

Fisher gained a bit of recent notoriety last month after his daughter, a Fox News correspondent covering the White House, shared a picture of him on social media and talked on air about how seeing him dressed in his protective gear at work made her feel.

But Fisher's more recent, inspiring thread on Twitter is intriguing for how it frames the risks for medical professionals handling coronavirus, and why he's continuing to do it.

As Fisher explained, he's now "genuinely more concerned" about his chance of dying while treating patients than he was "the day I launched on the space shuttle."

The reason: two shuttle flights out of 100 ended in disaster, meaning about 1.5 percent of shuttle astronauts perished in the attempt. But Fisher said he estimates his risk of death is between 9 and 12 percent if he gets COVID-19.

I know that might seem high, but as Fisher points out, it's the result of his age, and the fact that he has both diabetes and Type A blood, which are all risk factors.

"The death rate for people ages 70-79 in Italy has been 12.8%," he wrote. "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."

'A ship is safe in harbor'

Across the United States, reports say that it's not just a shortage of medical equipment like masks, protective equipment, and ventilators that's making it harder to fight Covid-19.

It's a shortage of doctors and other medical professionals.

In fact, in New York City the shortage is so acute that the city sent a text message alert to every cell phone in the city Friday, asking for all licensed healthcare workers to volunteer.

The situation in Texas isn't quite as dire, at least not yet, as New York. And perhaps that explains some of the pushback Fisher got -- both trolls questioning his math on survival rates, and then asking if he's so high risk, why would he be continuing to work at age 74?

His answer was inspiring and worth reading:

"Some of your comments question the wisdom of what I am doing. 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.

A ship is safe in harbor, but that's not what ships are for."

If it were easy, anybody could do it

We're in a strange and distressing time. Some people are sidelined, quarantined at home or even out of work now -- wanting to contribute while wondering what the future will hold.

Others -- maybe you -- are trying to keep business afloat, and planning for what might be left after the current crisis. As my colleague Cameron Albert-Deitch wrote yesterday, a survey from the U.S. Chamber of Commerce predicts that more than half of U.S. small businesses will have closed at least temporarily within the next two weeks. 

Against those odds, it can be tough to focus on the jobs at hand. But those 13 words at the end of Fisher's tweet are the key.

"A ship is safe in harbor, but that's not what ships are for."

If you're an entrepreneur or a business leader, you're continuing to build and lead not because it's safe or easy -- but because it's something not everyone else can do.

Sometimes it might seem like it would be easier to let somebody else make the hard decisions, and do the difficult work. But giving up or letting somebody else do it would seem wildly unnatural--it's just not what you're made for.