While no one wants to get swine flu, it would be particularly disastrous for the astronauts preparing to ride the space shuttle Discovery on May 11 for their mission to upgrade the Hubble Space Telescope.

So far, there's no indication that any astronauts have been exposed to swine flu, and NASA has not made any changes to operations because of the current situation, officials said.

But astronauts can get sick in space. In fact, microgravity appears to weaken the immune system, so NASA is careful to reduce the chances of spaceflyers catching nasty bugs before they lift off.

"NASA is cautious about exposing the crew to any and all viruses and bacteria in the preflight phase, whether that be swine flu or the common cold," said NASA spokesman Bill Jeffs. "NASA does not have to alter their current posture, because their current posture is already very conservative, as even a routine adenovirus [cause of the common cold] can be problematic on a space mission."

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Quarantine as usual

The seven members of Discovery's STS-125 crew are due to enter quarantine in less than a week to avoid catching any kind of sickness before their flight. This precaution is taken just before every mission, since the consequences of an astronaut getting sick are serious.

"Even a common cold can have a mission impact if a crewmember is not able to clear their ears due to congestion, especially with the changes in pressure that are required for a spacewalk," Jeffs told SPACE.com.

Astronauts usually undergo a physical exam called the "L-10" 10 days before launch. This exam includes swabs and other lab tests to make sure they're not already infected. After this time, NASA limits astronaut contact with other people, and formal quarantine starts at about seven days before launch.

Once the crew arrives at NASA’s Kennedy Space Center in Cape Canaveral, Fla., where shuttles lift off, the quarantine is strict, and the crew surgeon is isolated with the crew. The astronauts undergo a second exam two days prior to launch, and are given a brief medical check just before they suit up on launch day. Anyone on the ground who is sick or exhibits any signs of illness is prohibited from working with the crew.

Not foolproof

Even with such vigilance, astronauts have gotten sick during past missions. Wally Schirra came down with a cold in the middle of the 1968 Apollo 7 mission. Though nothing too serious resulted, Schirra did reportedly get irritable with Mission Control's requests and felt tired during the mission.

During 1968's Apollo 8 — the first manned mission to fly around the moon — astronaut Frank Borman suffered what may have been a bad reaction to a sleeping tablet. He discovered that vomit and diarrhea are not easy to clean up in the weightlessness of space.

In retrospect, some experts think Borman simply had space adaptation syndrome, or space sickness, which affects about a third of astronauts as their bodies try to adjust to microgravity.

German astronaut Hans Schlegel likely had a bad bout of space sickness that forced him to opt out of a scheduled spacewalk during the February 2008 shuttle mission.

And sometimes the atmosphere of the spacecraft can sicken astronauts. In 1999, high carbon dioxide levels in the Zarya module on the International Space Station might have been what made some space shuttle Discovery astronauts ill with headaches and nausea. The condition wasn't serious, though, and improved when the astronauts returned to Discovery.

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