A 2016 study into cardiovascular disease (CVD) amongst members of the NASA Apollo missions found that the CVD mortality rate in Apollo lunar astronauts was 4–5 times higher than in non-flight and Low Earth Orbit astronauts.
[…] although deep space radiation seems a likely cause underlying the higher proportional mortality rate due to CVD in Apollo astronauts, it remains unknown what specific factor(s) in the space environment is responsible."
Source : Apollo Lunar Astronauts Show Higher Cardiovascular Disease Mortality Scientific Reports 6, Article number: 29901 (2016)
Also see : Microgravity Immune Suppressionplugin-autotooltip__plain plugin-autotooltip_bigMicrogravity Immune Suppression
"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