Space and Infectious Disease
Life on Mars has never been confirmed, but that doesn't mean that it isn't there. It is possible that microbes live in aquifers deep under the surface, sources of water that would be necessary for human life. Exposure to these microbes would be novel, and all cells naive to the new pathogens. It is unclear how or if our bodies would be able to react to the new threats.
Another very real issue is that of Earthborne pathogens taking over and overwhelming the native flora, if such flora exists. Currently, the only inhabitants of Mars are rovers that have been baked at 500 degrees prior to their launch to prevent them from bringing bacteria to the Martian surface. This tactic doesn't work nearly as well with humans, and as such we need to work on ways to protect Mars from terrestrial microbes.
Another very real issue is that of Earthborne pathogens taking over and overwhelming the native flora, if such flora exists. Currently, the only inhabitants of Mars are rovers that have been baked at 500 degrees prior to their launch to prevent them from bringing bacteria to the Martian surface. This tactic doesn't work nearly as well with humans, and as such we need to work on ways to protect Mars from terrestrial microbes.