Miasma

Miasma is the concept of spiritual pollution. Many people will tell you that this means any contact with death, birth, and sexual activity, but this is untrue. It's moreso for people who committed murders of 'unacceptable' sorts by Ancient Greek standards. That said, you should wash your hands before doing ritual stuff anyway.

Murder, especially familial, and incest are usually the reasons for miasma. More on that here.