I agree with Rachel. I avoid sharing brushes. My horse Sonny is prone to fungus so he has his own set of brushes. I bought each of the horses I ride (there are 7, only 2 are mine) their own face brush. I wash my brushes once a month since some of my brushes (especially my favorites and go-to ones) are shared among all the horses. If you have a horse prone to rain rot then make sure she has her own saddle pads and blankets and wash them as often as you can.
hope this helped,
The bacterium that causes rain rot can be transmitted between horses. Avoid sharing blankets, brushes and saddle pads between horses and disinfect any equipment shared between horses. Clean your brushes will help prevent skin infections. Dirty, stale barns can provide a breeding ground for bacteria.