Agree, rain rot is contagious and can be spread from one horse to another. I love Vetericyn spray/gel for treating it. You'll know it's working if the oozy places dry up and if the grunge doesn't spread.
However, a lot of us are dealing with really wonky shedding this year. The hair looks dull and listless; it comes out in patches with near-baldness underneath, and it's symmetrical. In front of flanks and around shoulders are exactly where it's happening. Mine shed quite a bit around their ears, too, although I think it wasn't as bald as their bodies. I also thought it was rain rot at first but clues that it was not rain rot were - 1) symmetrical spots were shedding, which doesn't usually fit with rain rot, 2) skin was nearly bald but not scabby or oozy, which doesn't fit with rain rot; 3) it was not ouchy, which doesn't fit with rain rot; 4) it wasn't clumpy with hairs stuck together with goo/scab at the base. If you've got the same thing, it may just be atypical shedding due to a mild winter.