@fify-- Of course, it's contagious. You should be washing your clothes frequently with hot water. I would avoid wearing the same shirt twice. Towels and bed sheets have to be changed frequently as well.
I think the major cause of erythrasma is humidity, sweat. You need to keep your skin dry. Shower frequently and wear 100% cotton shirts. Cotton absorbs moisture. You can also apply baby powder around those areas to absorb sweat.