The plans are for astronauts to live for in space or on the moon for prolonged periods of time. However before humans can start this living in space we need to understand how our bodies will perform.
It is still unclear how our immune system is affected by living in space. However, scientists are now pointing out some relevant knowledge:
"...immune systems appear to become less capable in space, leaving them more susceptible to stowaway bacteria and viruses"
and
"...researchers studying the microbes that could infect astronauts recently found that at least one, salmonella, becomes significantly more virulent in weightlessness."
Clearly this needs to be understood to execute on the plan of further exploring space.