If there's one thing worse than getting sick, it's getting sick millions of miles from home. Unfortunately for Earth's vanguard of Martian explorers, that very well could be what's in store.

NASA researchers have just completed the first study of its kind on how the human immune system deals with a long-term stay in microgravity. The scientists analyzed a trio of blood tests from 23 astronauts who stayed on the International Space Station for missions of roughly 6 months. As they report in the science journal NPJ Microgravity, the scientists discovered that life in space moderately dampens your immune system, leaving your body more sensitive to "infectious disease, allergies and hyper-sensitivities," writes Brian Crucian, an immunologist at NASA Johnson Space Center, in the study. Other woes could include autoimmune issues, "altered wound healing," and the possibility of resurgent, older viruses, such as "latent herpes virus reactivation."

Stress test

Scientists have long known that astronauts suffer a surprising amount of immune system problem on short-term spaceflights. Take the 1968 Apollo 7 mission, when all three astronauts aboard developed severe colds. Several studies published in the past few years have isolated two problems in particular that keep astronauts off their A game. First, astronauts' T-cells, the ones that fight infection, sluggishly send delayed signals to one another when battling disease. And another important immune fighter, killer cells, battle intruders less energetically.

Here's the problem, though: Scientists were hesitant to say that these problems were really, unequivocally caused by being in space. After all, astronauts have one of the most demanding, risky, and stressful jobs on and off this planet. Stress can do serious harm to one's immune system, and short stays on the ISS can be especially grueling.

With their new study, Crucian and his colleagues believe they've found the first definitive evidence that it really is just being in space that's taking this toll on the human immune system. That's because this was the first study that analyzed immune antibodies at three points over a 6-month period: at the beginning, midpoint, and end of the mission.

But why?

The study agrees with what a lot of scientists has already found about microgravity and the immune system: Researchers who previously studied human T cells grown outside of a human body found that microgravity seems to stall them from getting to work when needed. Other studies analyzing fruit flies born on the ISS have found that microgravity alone essentially kills the immune response to fungal infection. (Weirdly enough, those same flies fought off fungal infection more effectively than Earth-bound ones at gravitational forces above Earth's. We're still not sure why.)

The next question is: Why? Crucian and his colleagues argue that it's hard to pinpoint a specific root cause. "Confinement, disrupted circadian rhythms, or the physiological stress associated with space flight itself," could all be working in sync to cause the immune effect, he writes.

This new finding certainly won't stop future long-term space missions, like a flight to Mars, but it is an important consideration for scientists planning them. Interplanetary travel brings with it a host of inherent problem, such as protecting astronauts from cosmic radiation, keeping them fed, keeping them safe, and bringing them home. Add keeping them from having a cold during the entire voyage to that list.

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