Fresh ginger. Not candied or crystallized ginger (the sugar is an immune-depressant). Get a piece about half the size of your palm, slice it as thinly as you can and put it in a medium pot full of water.

Boil. Turn down to low heat after it's boiled and let it simmer for 20 minutes.

By itself, it's fine, but it's helped immensly if you add the juice of half a lemon and a tablespoon of (preferably unpasturized) honey. I'm gonna stick my neck out out on the line here are posit that people who died of the flu had a bad diet and were eating uter crap and none of the garlic/chilis/ginger that does to much to abate the symptoms.