The dynamic of the spread of avian influenza viruses is complex and difficult to predict. However, the data received by the WOAH over the last 15 years helps to reveal a seasonal pattern: the number of outbreaks of HPAI usually is lowest in September, begins to rise in October, and peaks in February. Several factors can influence this dynamic, such as the wild bird migration pattern, unregulated trade, farming systems, biosecurity and immunity status.