The sunshine D-lemma.  Sunlight causes skin cancer, but it also produces vitamin D, a substance that seems to prevent some types of cancer and possibly other diseases.
D006801:Humans; D009369:Neoplasms / Q000517:prevention & control*; D012878:Skin Neoplasms / Q000209:etiology* / Q000517:prevention & control; D014466:Ultraviolet Rays / Q000009:adverse effects*; D014807:Vitamin D
D014807:Vitamin D