My grandpa swears that boiling a grapefruit - rind and all - in water produces something that will fight a cold off like no other. Last week I was sick and my parents have a tree bursting with grapefruit, so I tried it out. It was incredible, once I added some agave nectar.
I made it a few mornings in a row and even got a little exotic by adding some blueberries to the mix. Now I'm not sure that it punched out my cold, but it was a deliciously warm beverage for the morning.
Post Edit: At the request of Stephanie, here is an estimated exact recipe.
whole grapefruit, cut small wedges
other fruit (optional)
Dump ingredients into a pot, cover and boil on medium heat. Boil for 10-15mins or until grapefruit looks like the life has been boiled out of it. Enjoy.