A genetic quirk means thoughts of sex in certain people can brings on a fit of sneezing, scientists have found.
The inherited condition can affect both men and women and is thought to be closely linked to the way sunlight can make some people sneeze.
Dr Mahmood Bhutta, an ear, nose and throat specialist at the John Radcliffe Hospital in Oxford, launched the study after seeing a patient who suffered "uncontrollable" sneezing fits every time he had a sexual thought.
Sneezing usually occurs in response to nasal irritation, triggering a reflex that expels air at speeds of around 150 kilometres an hour.
Eyebrow-plucking can also provoke sneezing by stimulating the trigeminal nerve, which produces sensations in the face. But other more puzzling sneezing triggers are also known.
The "photic sneeze reflex" is an apparently inherited sneezing reaction to looking at bright sunlight, which affects almost a quarter of the population. More rarely, there were cases of people from the same family sneezing after meals.
Dr Bhutta wrote in the Journal of the Royal Society of Medicine: "It certainly seems odd, but I think this reflex demonstrates evolutionary relics in the wiring of a part of the nervous system called the autonomic nervous system."
"Sometimes the signals in this system get crossed, and I think this may be why some people sneeze when they think about sex."
"My husband has fits of repeated heavy sneezing regularly after starting to eat a meal. Not every meal, but it hasn't been possible yet to work out whether he will or won't start up or whether there are particular foods or other factors such as temperature which could work as a trigger. While in the throes, he is disabled for 10-15 minutes, with red, streaming eyes and nose, unable to see or talk, going through whole boxes of tissues very fast and has to abandon the meal. He's been doing this for years and I suppose we have both just grown used to his peculiar quirkiness. We were very interested to read of the sexual thoughts connection but he swears not in his case! It would be great to find out the particular cause/s for him and then perhaps be able to plan a strategy for prevention." - Ann Norman, Oxfordshire