[♩INTRO] If you set out to design the perfect method of disease transmission, you probably couldn’t do much better than a spaceship.

You’re crammed together in a small space with a bunch of other people. Every bit of air breathed out by one person is going to get breathed in by someone else.

As if that wasn’t bad enough, actually getting sick in space is way worse than getting sick here on Earth. It’s harder for your immune system to fight off the infection, and microbes can actually get stronger in space. So with NASA and companies like SpaceX finally thinking seriously about how to get humans to Mars, making sure people don’t get sick is pretty high on the list of priorities.

One problem is that weight and power are incredibly limited for anything that gets sent into space. That lack of power makes it pretty much impossible to use the kind of high-quality air filters found in places like hospitals. There’s another kind of filter that’s also not available on a spacecraft: the pull of gravity.

Particles in the air generally drift downwards, which is how that layer of dust on the top of your bookshelf keeps re-forming. It’s not just dust, though: imagine what happens when you sneeze in space! Instead of falling to the ground, everything you sneezed out just floats around waiting to be breathed in again.

Gross. To make matters worse, places like the International Space Station don’t really have separate rooms for different tasks. For example, one section contains the main control computers, a greenhouse, a treadmill, a stationary bike, two crew quarters, and the main cooking area!

That’s a recipe for a lot of germ exchange… All this is just the setup though, for what happens when those germs get inside the astronauts. For reasons researchers are only beginning to understand, simple organisms like bacteria get stronger in space, while complex ones like us get weaker. After just days in orbit, astronauts start losing muscle mass and bone density.

And, most critically, their immune systems start to weaken. That means cuts and bruises take longer to heal, and bone marrow doesn’t regenerate as easily. The terrifyingly named natural killer cells that help fight off infections — well, they kill less.

These symptoms aren’t unique to space travel. Submarine crews and antarctic explorers can end up with weakened immune systems, too. It’s what’s trying to kill you that makes space so much scarier.

While the humans are suffering, microbes seem downright thrilled to be up there. Bacteria cultivated on the ISS, for example, grow thicker cell walls than their siblings back on Earth. Which makes things like antibacterial soap less effective at killing them.

And not only do individual cells get stronger, the large-scale communities they create do, too! Some kinds of microbes build slimy, 3D structures called biofilms. Bacteria on the old Russian space station Mir formed biofilms so effectively that eventually nearly every available surface was covered in one.

Again, gross. Things like bacteria tend to get stronger in space because of the weightlessness. On Earth, things tend to settle out of a fluid in layers based on their density, which can separate microbes from their food.

But in microgravity, everything will stay more evenly mixed, which gives microbes easier access to nutrients. Everything in space is also exposed to a lot more DNA-altering radiation, which can make simple organisms mutate faster. In one study carried out on Mir, after just forty days in orbit, bacteria more than doubled their mutation rate.

That leads to a lot more variety for our immune systems to deal with. NASA and the other space agencies take these threats seriously. Astronauts are quarantined before they leave Earth, and almost everything they touch is sterilized before being sent to orbit.

The crews themselves are also composed of some of the healthiest human beings in the prime of their lives. Despite that, in a study of more than a hundred Space Shuttle flights, researchers identified 29 cases of infectious disease transmission between members of the crew. For missions lasting no more than about a week, this wasn’t that big of a deal.

And astronauts on the ISS always have capsules available to get them home if there’s an emergency. But imagine finding out one week into a year-long mission that you’ve got a serious infection and no chance to return to Earth. Once we figure out how to deal with that, we’ll be ready for our first long-term trip out into the Solar System.

Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon who help make this show possible. If you want to help us keep making episodes like this, you can go to patreon.com/scishow to learn more. And don’t forget to go to youtube.com/scishowspace and subscribe! [♩OUTRO]