Patients with illnesses even terminal ones have a much better survival rate if they are in a happy and relaxed state and frame of mind. Those with loved ones around them have been found to have the mental strength to fight their ailment. So laughter really is the best medicine!
Conversely, chazal say “tzchok v’kalus rosh motzi’in es ha’adam min ha’olam”. There is a world of a difference between tzchok or kalus rosh which is making fun, and laughter.