Fog reduces your ability to see road conditions and other drivers, and to be seen yourself. An expert rider will watch for fog in low lying areas and by lakes or rivers close to the road. Fog may be present at sunrise, sunset or other times when the air temperature is rapidly changing. As in rain, you will have to wipe your face shield or raise it to be able to see out.
In really heavy fog you will be able to see only a few metres. Any time you cannot see farther than your stopping distance, you are riding blind. If there is something stopped on the road, you will hit it. If you slow down enough to be able to stop in heavy fog conditions, you may very well be hit from behind, so either way you lose. The only smart thing to do is find a safe place to stop and wait for conditions to improve.