 She asked why the new vaccine was such a challenge. This parasite keeps changing its spots, if you like. It changes from the parasite that goes into your skin when you're bitten by a mosquito, rushes off to the liver within some hours, lives there for a week, different again. And then it comes out into your blood and expresses different antigen. So what's clearly happened is over millions of years, the parasite has evolved to learn to live in different tissues in our body.