Sadness acts pull for sickness, like a magnet attracts the metal. When you are happy, you have an armour around you which will protect you from illness. Heart has energy field, when it opens up. This energy field create some sort of charge around your body in which organisms dont get suitable environment.
In my experience with taking care of 100 people in WOW workshop at Humaniversity, Holland, I have seen so many times, when someone is sick, I talk to that person, do all medical check ups and analyse the situation and at the end I try to infuse some happiness into the patient - like talking about the same subject, making some jokes, giving some energy to their likes, uplifting them etc.
I used to do anything my heart says and I have seen amazing result that person is immediately out of his isolation created in the lieu of the sickness.