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.
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?