Thrush isn't contagious, but if the conditions caused it in one it might cause it in others. Some horses are more susceptible than others, though. Continuous wet and/or dirty conditions can cause it. Cracks in the frog can provide a home for it, even in conditions where a horse wouldn't normally develop it. And sometimes it just happens.
There are lots of commercial products to treat it. Lot of home remedies too. The good news is that there are lots of different ways to treat it -- that work. Iodine mixed with clove oil, iodine mixed with sugar and packed into all the cracks... hydrogen peroxide... Others here will, I'm sure, have more experience. But iodine + clove oil worked recently for a few horses at my barn.