Avian influenza viruses (H5N1 shown in green) emerging from birds and infecting a cell. A human strain (shown in purple / blue) infects the same cell. The genome segments (green and blue dashes) enter the nucleus (purple curved surface) and are copied. The new copies exit the nucleus but get jumbled together and form the genome of a new viral strain (red-yellow virions) that might be as lethal as the bird flu and as easily spread from person to person as the human flu. This seems to be the recipe for the next influenza pandemic.