Healthcare workers battling the coronavirus pandemic just got an off-Earth shout-out on National Doctors' Day.

NASA astronaut Drew Morgan, a medical doctor who's one of the three people currently living aboard the International Space Station (ISS), tweeted his appreciation today (March 30) to the brave doctors, nurses, medical technicians and other people on the front lines of the war against the novel coronavirus and the disease it causes, COVID-19 .

"As a medical doctor looking back on our planet on #NationalDoctorsDay, I think of the healthcare professionals & volunteers that are risking their lives in this crisis. We're at our best when we help each other. I'm in awe of your selfless service. Thank you from @Space_Station," Morgan said via Twitter today . This was the text accompanying a photo of himself and a floating stethoscope in the station's cupola, with Earth in the background.

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As a medical doctor looking back on our planet on #NationalDoctorsDay, I think of the healthcare professionals & volunteers that are risking their lives in this crisis. We're at our best when we help each other. I'm in awe of your selfless service. Thank you from @Space_Station. pic.twitter.com/P1xoRftbSOMarch 30, 2020

Healthcare workers are indeed risking their lives to treat COVID-19 patients. For example, medical professionals make up almost 14% of Spain's confirmed coronavirus cases, according to the New York Times . And COVID-19 is raging through the healthcare workforce in New York City, which has been hit particularly hard by the outbreak. A "doctor at a major New York City hospital described it as 'a petri dish,' where more than 200 workers had fallen sick," the Times' Michael Schwirtz wrote in a story today .

As of today, there are more than 740,000 confirmed coronavirus cases worldwide and more than 35,000 COVID-19 deaths, according to data compiled by Johns Hopkins University .

Morgan's two companions aboard the ISS are fellow NASA astronaut Jessica Meir and cosmonaut Oleg Skripochka, commander of the current Expedition 62. The trio will soon be joined by three colleagues: NASA astronaut Chris Cassidy and cosmonauts Anatoly Ivanishin and Ivan Vagner are scheduled to launch toward the orbiting lab aboard a Russian Soyuz spacecraft on April 9.

National Doctors' Day is celebrated in the United States every year on March 30 (though some countries use a different date). The first such observance was held in Georgia in 1933, according to the website doctorsday.org .

"On March 30, 1958, a Resolution Commemorating Doctors' Day was adopted by the United States House of Representatives," the website reads. "In 1990, legislation was introduced in the House and Senate to establish a national Doctors' Day. Following overwhelming approval by the United States Senate and the House of Representatives, on Oct. 30, 1990, President George Bush signed S.J. RES. #366 (which became Public Law 101-473) designating March 30 as 'National Doctors' Day.'"