Some are «systemic» reactions that they affect the whole body, others cause damage to the immune system and can lead to chronic health problems. (raisingrascal.com)
Their illnesses could be caused by damage to their immune systems, or by copies of the virus that have hung on in their bodies. (newscientist.com)
The extra radiation in space also weakens the immune system, and it is possible that microgravity and radiation cause more damage to the immune system together than they do separately. (newscientist.com)