A food scientist has warned against eating supermarket salad leaves, as they're often laced with a bug that triggers extreme vomiting and diarrhoea - and can prove lethal in vulnerable Britons. 'Salads, lettuce and side salad infections can kill', Dr Primrose Freestone, a microbiologist at the University of Leicester told The Times. 'I wouldn't touch a bagged salad in a million years.' Germs use juices from bagged salad leaves to become more virulent. Firstly, all pre-packed supermarket salad leaves should be washed, even if they're labelled 'washed and ready to eat,' she said.