One rather odd one I didn't expect to hear quite so much about this year is how an onion will "draw out the illness" and turn black. I am a firm believer in the placebo effect, as well as the importance of optimism on patient health and outcome. Still!
Snopes has a page explaining the origin of the myth; apparently, the superstition has been around for over 100 years. Not only is there no scientific evidence, there are no studies because it's SUCH a ridiculous claim! (It would go into the same category as testing unicorn horns for a cancer cure.) Snopes quotes a Wall Street Journal article:
"Biologists say it's highly implausible that onions could attract a flu virus as a bug zapper traps flies. Viruses require a living host to replicate and can't propel themselves out of a body and across a room."