The effects of spaceflight on the human body have been studied since 20th century. There is enough evidence that microgravity influences metabolism, heat regulation, heart rhythm, respiratory system, muscle tone, and bone density.
More recently, studies show that stressors encountered during spaceflight (e.g. microgravity, radiation, chronic stress) weaken the immune system and provide a positive environment for some pathogens. The fact that the immune system reacts like this when the body is infected can be explained by the fact that the body doesn’t know what to do and this is its attempt to turn on all possible defence systems.
Space offers a possibility to get a better understanding what causes diseases. The expectation is that when it is clear which pathways are affected by weightlessness, it will be possible to find ways to prevent and cure these processes.