User blog comment:Mckrongs/Monster Hunter 4 for the Nintendo 3DS/@comment-4694695-20130830104825/@comment-11537809-20130830110438

It stops you from naturally recovering (regaining the red part of the hp bar when you get hit) and decreases your defense. However, if you inflict enough damage to the monster that infected you, you will be free from the virus and gain a boost in affinity (more crits). On monsters, the virus will revive them upon dying and allow them to pass the virus on to hunters (correct me if I'm wrong)