Right, that article doesn't. I saw several articles which do mention it, for example https://www.statnews.com/2019/02/15/scott-kelly-nasa-twins-study-immune-sys

Right, that article doesn't. I saw several articles which do mention it, for example https://www.statnews.com/2019/02/15/scott-kelly-nasa-twins-study-immune-system/ https://www.statnews.com/2019/02/15/scott-kelly-nasa-twins-s... "Immune issues sound familiar to Dr. Jerry Linenger, an American astronaut who spent more than four months on the Russian space station Mir. He said he was never sick in orbit, but once he came back to Earth “I was probably more sick than I was in my life.” Astronauts launch into orbit with their own germs and get exposed to their crewmates’ germs and then after a week with nothing else new in the “very sterile environment” of a space station “your immune system is really not challenged,” Linenger said." Thus we can see that not being sick while in orbit, despite all the stress and other space factors - microgravity/radiation/etc. - which are supposed to weaken immune system, while getting sick on return when those space factors disappear clearly favors the immune system antigen exposure dependence explanation.