This article in NewScientistSpace.com talks about research into why astronauts are more prone to infections. They now have data that suggests that microgravity interferes with the activation of T-cells. This explains why NASA needed to institute 10-day quarantines for all of their astronauts. Worse, the article concludes with this:
No one is yet sure whether an astronaut with a suppressed immune system would be able to complete a three-year space mission – the estimated time for a round-trip to Mars. Astronauts and cosmonauts live aboard the International Space Station in six-month shifts.
Dammit. Now to discover if simulated gravity is good enough to do the trick or if you need a bona-fide pile of mass nearby to curve space-time. Let’s hope the Habitrail Solution is good enough.