The Simian Flu is explained in this new short film that attempts to link the events after Planet of the Apes to its sequel.
The premise of Dawn of the Planet of the Apes is set ten years after the events of Planet of the Apes. If you recall, the film ended with the revelation that the world was about to contend with an outbreak of the Simian Flu. Well, the sequel continues with that story a decade on after the disease nearly wipes out humanity.
But now, a series of shorts have surfaced that attempts to link back the events that bridge both films. The first of which takes place in year one after the outbreak of the Simian Flu.