It's entirely possible. Given what Cosmo explained, it could have something do with worlds or the consequences of having too many universes overlapping at one point...
Every living thing has some sort of immune system. It doesn't have to be internal, it can be external, etc. Each of us come from different places, which means we are each used to different things like food and chemicals from our native universes. But my universe may not have something that your universe does. If I were to come into contact with something that my immune system doesn't know how to handle, then the reaction from the system will be different. Something that is native to your world might for instance cause some of my organs to shut down as I've never been exposed before and my body doesn't know how to handle or fight it off.