Researchers have to visit people who are reported sick, test them to make sure they were infected by the specific strain being investigated, and then question them intensively about what they ate or drank days and weeks before.
Demi said: “I have done ballet since I was a little girl and a huge part of my eating disorder came from that.
“At about 15, I started to realise that a lot of dancers are very, very thin. I felt they got a lot further and got a lot more attention.
“I was so passionate about ballet, I’d have done anything to get where I wanted to be.”