Avian influenza normally spreads in birds but can also infect humans. Human infections are primarily acquired through direct contact with infected poultry or contaminated environments.

While recently-identified avian influenza viruses do not currently transmit easily from person to person, the ongoing circulation of these viruses in poultry is concerning, as these viruses cause severe disease in humans and have the potential to mutate to become more contagious between people. WHO's Health Emergencies Programme therefore monitors human cases of avian influenza on an ongoing basis.