Everyone is allowed to take breaks for when they are sick. Just like people take off from school or work, its no different from taking off a day from writing because your sick.
I get pretty bad headaches and i dont write till they go away, because pushing out words just to get to a quota when your not feeling well usually doesnt help the writing.
Although when i'm really sick, i just lay in bed and write. Its one of those things i do because i'm not going to do schoolwork when i'm sick so its like a good make up time for writing!