My grandmother's recommended cure for a cough was to crush up eggshells, soak them in brandy overnight, and then strain and drink the brandy. Possibly there might be some helpful protein or calcium compound that leaches out. Or it might be the brandy that does the trick by itself.
I find putting a dollop of Vicks into a pot of water taken off the stove and inhaling the steam (or even just putting it into a sink of hot water), does a good job of decongestion.
Of course, neither of these "cure" influenza, they just make the symptoms of the infection easier to bear.
My grandmother had an interesting book of remedies from the early 1900s. Apparently, a good cup of beef broth was helpful, if not a perfect cure, for all ailments up to and including Black Death. (I cannot explain the authors' criminal disregard of chicken soup.)