As the virus’s basic reproductive number increased, so did the frequency and scale of the outbreaks it caused. Between 1959 and 1992, outbreaks of deadly avian influenza had occurred roughly every three years. Most affected fewer than five hundred thousand birds. Between 1993 and 2002, outbreaks occurred once every year, and between 2002 and 2006, they occurred every ten months. About half of these outbreaks affected millions of birds at a time.47
