the solution is to regularly flush a toilet that is not used often,to pour several cups of household bleach into the overflow pipe of the flush valve inside the tank. This will disinfect the rim of the bowl,requires running a snake down through the vent pipe from the roof. Climbing around on a roof is tricky and best left to a professional plumber who has the proper equipment and experience.The last wash the ceramic surface to be repaired with soap, warm water and a sponge. towel dry.