User blog comment:Mckrongs/Monster Hunter 4 for the Nintendo 3DS/@comment-6050823-20130806163748/@comment-6050823-20130806191527

They aren't invincible, they just take less damage. The virus can affect you in two ways, if your not able to attack or do enough damage, you will be affected by the virus and it will cause the red part of your health to disappear(Pretty much you won't be able to rengenrate health). If you are able to do enough damage, you will gain an immunity to the virus while also getting a massive affinity boost.