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Why and How Do Astronauts Get Sick in Space?

Spaceflight presents a myriad of challenges for the human body and health. Learn how experts are working to lessen the side effects for future astronauts.

By Katie Liu
Oct 16, 2023 3:00 PMOct 23, 2023 1:25 PM
Medium shot of a female astronaut testing spacesuit camera and listening to instructions during pre-flight preparation
A female astronaut testing spacesuit camera and listening to instructions during pre-flight preparation (Credit: Frame Stock Footage/Shutterstock)


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Apollo 7 sent a trio of astronauts into space in 1968, making it the first crewed flight of the Apollo Program. But about 15 hours into the flight, astronaut Wally Schirra began developing the symptoms of a bad head cold, with the rest of his team following suit. Balancing a fog of symptoms, including sniffles and congestion, on top of their heavy workload was a recipe for misery.

Considering the extensive training and screening astronauts go through to ensure they’re in optimal health for missions, it may seem ironic that they’re also more likely to get sick while in space. Luckily, researchers are working hard to understand the ways and reasons why astronauts get sick, as well as how to mitigate those risks for present and future space exploration.

What Happens in Outer Space?

Different kinds of clinical events manifest in space, according to NASA immunologist Brian Crucian. There are colds, sore throats, abnormal allergies and even skin rashes – an unexpected outcome in otherwise healthy individuals.

Scientists at NASA’s Human Research Program (HRP), in the Johnson Space Center located in Houston are investigating the reasons why organ functions shift in space and the root cause behind these symptoms.

What Are the Side Effects of Being in Space?

Among the hazards of spaceflight is cosmic radiation, microgravity and psychological isolation, and they come with mean side effects. For example, human bodies evolved in an environment of constant gravitational force. Take that away in space, and many of the stressors we’re working against daily are gone. This means the heart works less hard, and blood is no longer pulled toward the bottom of our bodies.

Astronauts then experience what’s akin to aging on Earth. Muscles atrophy, as do bones. Radiation exposes astronauts to the risk of developing cancer later. In orbit, sunrises and sunsets fly by about 15 times, so there’s no clear way of maintaining usual circadian rhythms without intervention.

That’s not to mention the nausea-inducing motion sickness attributed to something called Space Adaptation Syndrome.

Read More: Spending Time In Space Can Harm The Human Body

How Is Our Immune System in Space?

Because space is not the place to have medical emergencies, experts are researching how spaceflight affects the immune system.

“All kinds of strange things happen in spaceflight, which a normal human person or human physiology is not used to,” says Satish Mehta, a virologist at JES Tech.

One strange effect is how the cells of the immune system operate in general. T-cells and natural killer (NK) cells are the defenders of our immune system, which reduce in function during spaceflight. That reduction can reactive normally latent viruses. A dampened immune system also leads to mild illnesses, manifestation of allergies, and even rashes, which have been observed in a few astronaut cases.

A recent study by Odette Laneuville, an associate professor at the University of Ottawa, also examines the immune issue. After testing astronaut blood aboard the ISS, sampled before, during, and after their six-month sojourns, Laneuville and her team found that certain immune-related genes, about 100 of them, decline in activity when in space.

One upside is that while immune system productivity dips in space, it eventually returns to its normal state once astronauts return to Earth. However, it’s the being in space that scientists are grappling with.

Why Does the Immune System Change in Space?

Being in a new environment means the body has to adapt quickly. The unique combination of microgravity and exposure to space radiation, for example, calls for the immune system to rapidly adjust to new circumstances.


In space, since gravity no longer pulls blood and fluids to our legs, it all shifts headward. The body’s response is to lower its blood volume to restore homeostasis. One explanation for the downregulation of immune-related gene activity observed in Laneuville’s study is that the number of circulating red and white blood cells must also decrease – as there’s too many of those cells in a smaller amount of blood. This in turn would impact how the immune system defends itself.


Stress is also an additional factor. Space missions have high stakes, from launch to landing. Experiencing constant stress in a physically and socially isolated environment, away from loved ones, where many of our natural rhythms are disrupted, places heavy burdens on the body.

Travel Time

The length of the flight also bears weight on the astronauts. Most short-duration space flights take place on the shuttle, and launch itself is a huge stressor. It takes time for bodies to acclimate to space, but since the shuttle missions are so short, astronauts can’t adapt.

“It can be 30 to 45 days – it varies – until crew members get to what we call ‘space normal,’” Crucian says. “The short-duration missions never really get out of that phase.”

Long-duration missions, on the other hand, typically take place over six months, which is enough time for astronauts to settle in.

Ultimately, getting sick in space isn’t isolated to one simple cause, according to Douglass Diak, who is an exercise immunologist and virologist at Aegis Aerospace Inc.

“It’s a constant, chronic stress on the body that is causing a dampened immune system and a latent viral reactivation – normally asymptomatically,” Diak says. “But we use it as a biomarker to show that the immune system is losing the ability to control these as it normally would on Earth.”

Read More: Space Missions Make Lasting Changes to Astronauts' Brain

Physical Health Is not the Only Thing at Risk

To state the obvious: When our bodies feel bad, we feel bad, too. Apollo 7 also showed how space can impact mental health, according to Jennifer Levasseur, a curator at the Smithsonian’s National Air and Space Museum.

“The other thing to think about in terms of illness is how it affects one’s mental performance and psychological and emotional state,” Levasseur says. “Being sick can really be a detriment in a place where you have to be performing constantly. There’s not really a time when you can’t be prepared.”

Plus, being away from family and friends is hard. The ISS is about 250 miles above sea level – just a tad longer than a day trip from Houston to Dallas – but there are no quick ways home from space.

She also points out a study done by NASA in 2010, which analyzed 10 astronauts’ journals, allowing for a deep (and anonymous) dive into their psyche, morale, and behavior over long-duration spaceflight. Crew members wrote about things spanning from their workloads, to interpersonal relationships and conflicts, to food frustrations.

“We are getting tired of eating chicken all the time, but it will be OK,” one such astronaut wrote.

“The idea of wellness in space is very holistic,” Levasseur says. “It’s not just about the physical being, because there are so many different ways that we know our mental and physical health play off each other.”

How to Take Care of Yourself in Space

A lot of the countermeasures NASA is developing to protect astronauts aren’t necessarily space exclusive. They prescribe practices like daily exercise, vitamin supplements, and a clean living environment.

“The countermeasures that are employed, and that we’re looking at here at JSC, are what you would honestly think of here on Earth: improvement in nutrition, altered diets, increasing macronutrients, more fruits and vegetables,” Diak says. “Things that your doctor would basically say to you to help you here on Earth.”

Exercise is key. Working out can move the immune cells in our peripheral tissues into our bloodstream. With more immune cells patrolling around, there’s a higher chance of catching illnesses. On top of that, exercise maintains muscle and bone, which are at risk when in space.

On the mental side of things, astronauts get downtime outside of their working schedules. Exercise can offer a mental break, but astronauts also have designated times to contact their families back home. Levasseur says they can also bring books or hard drives with their favorite shows up to the station.

“It’s a myriad of problems that astronauts have to deal with,” Diak sums up. “So we have to counteract that with a myriad of solutions.”

Read More: Back Pain is Highly Common Among Astronauts

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