Alcohol
I know this is sort of controversal, however, there have been medical reports that people who have at least a drink a day (whom are otherwise healthy) do get less illnesses, such as flu, colds, etc. This is not to say that their liver is going to die, however, they do decrease the amount of communicable illnesses that people who do not drink get.
If you search the net, it's difficult to find because you know everyone now is trying to prevent drinking (as the US is now becoming increasingly communist)--this is something I learned in one of my college classes while doing a paper and searching their online library.
So really, you are not far off in saying that internal alcohol could prevent this illness.
Sherry