Soap cancer: a soft, schmooshy area on the bottom of a bar of soap that, once it begins, cannot be cured.
Towel AIDS: a strange, metallic smell that becomes noticeable on damp towels after they have been used for a while and stored on a metal towel-holder.
Shower-curtain herpes: crusty spots that appear on shower-curtains. These spots can be scraped off but they will appear again within a month.