Worms

Thread worms, to be specific. So called because they are thin and thread-like.
I'm not sure how you get them, but I did in the summer of 1985. We were on holiday in Cornwall at the time. Me, my Mum and Dad, and my sisters (my brother was too grown up to go on holiday with his "totally boring parents" at that age.)
It was a nice enough holiday. We were staying in a pretty fancy hotel. I played tennis against my Dad in the daytime, and at night I wandered into Newquay and forlornly tried to meet a girl.
Then I noticed that my bottom was really itchy all the time.
Then I went for a poo and noticed that there were some white, thread-like objects moving about in my deposits.
I thought, "That doesn't look right."
So as Bob Geldof was saving Africa, and Phil Collins was flying across the Atlantic in a helicopter with a cheap suit and a receding hair-line, I was sitting in the bar of a nice hotel saying to my Mum, "I think I've got worms."
She said, "Don't worry."
And she got me some tablets the next day, and that was the end of that.