Can i take effective herbal therapies to prevent a legionnaires' disease?

Nothing approved: Although, you may find claims on the web of people having taken "natural" supplement/herb that helped them cure legionnaire's, there's nothing approved or shown to be effective in studies. Given the fact that it may become fatal in certain cases if left untreated, i'd strongly suggest prompt medical attention and institution of a commonly accepted treatment.

None known: None that we know of. http://en.wikipedia.org/wiki/legionellosis.