The number of swine flu cases is rising rapidly and experts are warning in the worst-case scenario 30% - or one in three of the UK population - could become infected and up to 65,000 people could die this winter. These sound like big, scary numbers, but what do they actually mean and how have they been calculated?
A new strain of swine flu - influenza A (H1N1) - that is spreading from person to person has sparked concerns that the outbreak is the start of a pandemic. Nature reports on the latest developments and examines what might be done to combat the threat.