Viruses and bacteria, having genetic material, have the ability to change that genetic material occasionally. This could be harmless or it could mean that those looking to combat those pathogens are going to have to start all over from scratch, or they've become much more spreadable or deadly. It's adding in these mutations into Pandemic: Hot Zone - Europe, that we get a look at here.
From the preview:
Pandemic: Hot Zone - North America introduced Crisis Cards to give players new and unexpected challenges to overcome. In that same vein, Pandemic: Hot Zone - Europe brings forth a new challenge known as Mutation Cards with lasting effects that increase the danger of the viruses in play.