It actually has been a little while since this has come up. For those here six months ago, there is probably a collective groan though. ;)

If you saw the actual worm hanging out the fish's anal vent, here are two pictures of it so you can identify it as a Camallanus worm or not.

http://www.rhusmann.de/aqua/bilder/camallan.jpg

http://www.max-discus-dream.de/mddnew/Images/Erreger/camallanus3.gif

If it is, click to the Guppylog Quick Links fast and read all of the entries under Camallanus. Your tank will need to be treated with a dewormer. Don't take anything (hands, nets, fish, siphons) from that tank and out them in another tank. The good news is that most of the fish will respond positively to treatment, though the one with the worm may not survive the process.

I really hope I'm wrong. But for the sake of your guppies, please take a close look at that object extending from that one animal and compare it to those references - quickly.

If that is a Camallanus worm spreading youngsters, unless you are on the West Coast, the stores are probably close to closing or closed and you can't get a dewormer tonight. Read the Quick Links. get a good night's sleep, and plan on getting a dewormer tomorrow and treating the tank when you get home.

Hope I'm wrong. That is one of the first questions though to ask in such a situation.

When I was a kid and spent more time watching the fish than corresponding about them, I recall the shock that accompanied espying a female guppy that  had swallowed a human hair a little over an inch long! Perhaps she mistook it for a Dero worm. Thought surely she would die. Over the course of a day or so, that hair began to work though her. It really was gross watching the hair go into her mouth and extend out her vent! Somehow the guppy's intestine contorted enough to let that hair go through.

Of course parents of small children and keepers of over inquisitive dogs could have told me, "all things must pass." ;)