Geoff Watts explores the incredible ecosystem we harbour inside our intestines.
In the name of science, he has his stool sample analysed to find out how many bacteria live inside his guts.
"It's a veritable zoo," says Prof Peter Boriello of the Health Protection Agency, "in fact there are more bacteria inside your gut than people who have ever walked the planet".
Some are good, some are bad and most are downright ugly.
Trendy new probiotic supplements promise to boost our beneficial bacteria, but can a culture a day really keep the doctor away?