Taking a hot bath can only help you clear up a cold or flu if the bath water temp is able to actually raise your body temperature high enough that you are sweating.
Taking hot steamy showers help me get rid of my colds and opens up my sinuses to help me get rid of my stuffy nose.
When I have a cold or my nose is stuffed up I turn the shower water on to as hot as water temperature as I can stand it and the hot steamy water gets rid of my stuffy nose really quick and I feel much better.