Human papilloma viruses, which are the commonest sexually transmitted pathogens, come and go from the genital tract and usually do little harm except for types 16 and 18, which cause the great majority of cervical cancers. Two thousand US women between the ages of 14 and 59 in the latest round of the National Health And Nutrition Examination Survey (NHANES) provided self-collected vaginal swabs, and over a quarter of them carried some kind of HPV, though the pathogenic strains are relatively uncommon.

Studies of post-traumatic stress disorder have given mixed messages, with “counselling