Imagine there was a nasty disease that affected 1 in 100 people. And imagine that someone invented a drug which treated it reasonably well. Good work, surely.
Now imagine that, for some reason, people decided that 10% of the population need to be taking this drug, instead of 1%. So sales of the drug sky-rocket. Eventually some clever person comes along and asks "This is one of the biggest selling drugs in the world - but does it work?" They look into it, and find that it doesn't work very well at all. For about 9 out of 10 people, it's completely useless! What a crap drug.
