If you haven't already tried, do a search in Google, typing in 'human parasitic infection' and a plethora of information (and photos!) on the subject becomes available. Apparently thread worms are fairly common in humans - see web.ukonline.co.uk/ruth.livingstone/little/thread.htm - though the photo isn't for those of a queasy disposition! Seriously though, if you think you or your housemate could have some kind of infection (have you eaten anything dodgy recently or visited anywhere intrepid where such infections are widespread?) go and see your GP. You may also need to warn friends/ family with whom you've come into close contact with as it seems the eggs of many of these parasites can be easily transferred if you are not scrupulously hygienic after going to the loo.