You don't have to be afraid of rot.
It does very little damage, you will get healed from rot during a main quest
and can get the ability to handle Tenebrium without catching rot through a sidequest
get some Tenebrium for Brandon in Silverglen
or use a book that increases your Tenebrium Skill
Geomancer's shop in Sacred Stone.
If you absolutely want to get rid of an existing rot infection, you can also