Elements of a flu pandemic

A human influenza pandemic generally occurs every 30-40 years, and many experts believe that the next pandemic is overdue. There are a number of requirements for a new pandemic to begin: i) emergence of a new viral strain to which general populations lack sufficient immunity, ii) ability of this new strain to infect humans and to cause severe disease, and, iii) capability to transmit efficiently among humans, i.e., be highly contagious. For a pandemic strain to establish at a global level it also requires a vehicle for worldwide spread.