“Spaceflight causes alterations in human immunity, a finding which has been well documented immediately following spaceflight. Limited in-flight studies have also confirmed that to some degree immunity is compromised during spaceflight. A comprehensive understanding of the nature of these immune changes is lacking.”
Currently, astronauts returning from says at the International Space Station are routinely quarantined until their immune system has recovered sufficiently.
“Numerous studies have indicated that spaceflight results in dysregulation of the human immune system. This phenomenon has been well documented following flight, where landing and readaptation are significant stressors. More recent data, particularly studies onboard ISS, indicate that this phenomenon persists for the duration of a 6-month orbital spaceflight. Persistent decrements in the function of specific innate or adaptive immune cells, or alterations in cytokine production profiles, could result in clinical risk to crewmembers participating in exploration-class space missions.”
There is as yet no (agreed) explanation as to why the human immune system becomes suppressed in microgravity environments.