Like all Inhumans, Medusa's immune system is weaker than that of an average human. However, due to her frequent ventures into the outside world, Medusa's immune system has been strengthened so much that it is no longer as weak as that of her fellow Inhumans. As such she has attained a resistance to the pollutants of the outside world, which enabled her to join the Fantastic Four (as well as be a former member of the Frightful Four) in the first place.