Equine influenza is highly contagious and spreads rapidly among naive horses. Horses 1–5 yr old are the most susceptible to infection. Orthomyxovirus A/equine-2 was first recognized in 1963 as a cause of widespread epidemics and has subsequently become endemic in many countries, except for New Zealand and Iceland. China, Japan, and Australia experienced devastating epidemics of equine influenza affecting tens of thousands of horses in 2007. Equine influenza had not been reported in China since 1993, in Japan since 1972, and had never been reported in Australia.