I had a serious problem with seasonal (May) laryngitis for a number of years in the late '80s caused (I eventually found out) by mould or mildew in the "ventilation system" (I use the term advisedly) of the company I was working for at the time. When I left there, the problem was solved.
My voice still is suseptible to colds -- usually just before pre-Christmas gigs. The best advice I ever received and what is the only thing that really seems to work in the long run is keep hydrated -- drink lots and lots of water -- and get plenty of rest.
The voice comes back.
Short term, well, "Herbal Mist", "Singer's Saving Grace" or similar throat sprays available at health food stores.
Linn the Bat Goddess