Why do willow trees make an Aspirin precursor?

We posed this question to Professor Peter Rothwell from the University of Oxford...

Peter - That's a very good question. It looks very much as though a lot of different plants synthesise salicylic acid. It's very effective in warding off infections and treating particularly fungal infections in plants. So it looks as though it's synthesised for that purpose. And in fact, one of the downsides of modern food production is that we get a lot less salicylic acid from our diet because all of our food now is pristine with no infection.