Of getting hiv/hepatitis-b/c by toilet water splashing back at your hemmorhoid/anal fischure if last person was infected & didn't flush completely?

Forget it: That's not a realistic concern. During the old days, family members of patients simply did not get infected with these except be practices recognized as risky. If you find yourself thinking obsessively about this stuff, a short course of cognitive behavioral therapy may be a huge help. Life is hard enough without such fears. Best wishes.