It's easier to get sick in a zero-gravity environment, and it seems that the cause is genetic.
Researchers at the San Francisco VA Medical Center have identified a set of immune-response genes that do not turn on in a weightless environment. SFVAMC biochemist and former astronaut Millie Hughes-Fulford, has found that a signaling pathway called PKA will stimulate 99 genes to produce T-cells, essentially for human immune functions, when in a regular, Earth-bound gravity field.
In the absence of gravity, 91 of those genes remain domant and the remaining eight, although functional, are significantly inhibited.
The study helps solve the 40-year old riddle why the immune system becomes so impaired in space. Hughes-Fulford pointed out that there are only two known situations in which T-cell function becomes so severely compromised: HIV infection and weightlessness.
The research was conducted on human immune cells in culture housed in a device called a random positioning machine, which simulates freefall.