A bad new strain of influenza is among the top disease threats that experts are watching for, even as the world attempts to bring to an end the current coronavirus pandemic. The last pandemic flu the world encountered - the swine flu outbreak of 2009 that began in Mexico - was less deadly than initially feared, largely because many older people had some immunity to it, probably because of its similarity to other flu viruses that had circulated years before. The new flu strain that has been identified in China is similar to 2009 swine flu, but with some new changes.