No it is not contagious... however.... poor water quality, over crowding and over feeding causing stress to tank inhabitants makes all the fish susceptible as it is a bacteria normally found in a tank. The wrong fish kept together can result in fin nipping and once you have an open sore or tear in a fin the bacteria takes hold. A second fish gets a tear and the bacteria shows up again. Make sense? Hope that helps.
So don't over crowd, over feed, and lots of water changes. Keep your water pristine. That is the best medicine!