The latest Falcon 9/Dragon resupply mission to ISS contains a small experiment to evaluate the effect of space radiation in a habitat on antibody encoding genes. This goes directly to one of the Grand Challenges of space settlement.
Not only is the human immune system mildly suppressed by long stays in orbit, but the virulence of some bacterial diseases gets worse. Also, medications and nutrients in food break down somewhat faster in orbit due to radiation. This could lead to a cross-over where our immune systems would not cope well with spaceflight.
The secondary issue is the cause of these problems. With the human body, microgravity is a major factor, along with radiation. With medications and food, radiation is the only factor. With bacterial virulence, it may be one or both.