Yoghurt would be completely ineffective against giardia, which is a delightful little parasitic beastie that makes new baby beasties every 3 days in your intestines. The result is yellow frothy diarrhoea, dreadful pain, fever and rapid loss of weight. Antibiotics kill it (you hope) but it can take more than one course. The yoghurt is for afterwards, to restore good bacteria so that normal digestion can continue. My husband has had dysentery, and although he's black he turned as pale as a ghost before it was dealt with. There really are some horrid things out there, just waiting to move in to your cosy warm tum! The worst thing about the giardia was that I had to sit on a plane from Ghana to Heathrow for several hours, trying in vain not to explode before I reached the loo.