I realised something unusual had happened as soon as I entered the lab: dead cotton rats littered the floors of many of the cages. I hadn’t expected fatalities so early. The team had only given them the flu virus the day before and we thought it would be a few days before they developed symptoms. The powers that be had told us the virus came from South East Asia but that could mean a lot of things. It might be a natural mutation, or it might be of Chinese Government manufacture. It made little difference to us, our job was to assess it not trace it, and epidemiologists use cotton rats because they’re a good model for studying human influenza.