It can happen easy the most common way is by introducing it by a sick fish or by letting water from the LFS or someone elses tank getting into yours.
The only way to treat it in a tank with anytype of invert or coral is to setup a QT tank. Use the search button and type in QT and in the user type in Beth that is really good info. Or I think its at the top of the this forum page in the stickie.