Dormant viruses activate during spaceflight. Herpes viruses reactivate in more than half of crew aboard Space Shuttle and International Space Station missions. While only a small proportion develop symptoms, virus reactivation rates increase with spaceflight duration and could present a significant health risk on missions to Mars and beyond. Source 6w.
An expected result, we do not have all the technology necessary to prevent specific viruses from attacking plus other micro organisms that can take over. After all, a lot of viruses and bacteria comes from no other than space itself.