93% of flip-flops worn over the course of a summer will end up with fecal matter on them. 20% will have E. Coli. Staph was also found. And "feces, urine, spit, vomit, animal droppings were all present." Furthermore, the film of grime (you know, the "tan line of dirt") that coats your feet at the end of a day of flip-flopping around town is some dangerous stuff, and if you happen to have a cut or open sore on your foot, you could actually be in danger of getting a serious infection.
So if you flip-flop, consider washing your feet upon arriving home, or do like I do and take an all-purpose Sani-Kids wipe to your grimy piggies.