The Dog and Cat Plague refers to an event that takes place in the continuity of the original Planet of the Apes film series.
In the year 1983, NASA astronauts returning from space, brought with them a virus that eradicated all dogs and cats, seemingly overnight. Humans as well as most species of Apes were immune to the virus. Humanity, in their overwhelming need for pets, turned instead towards keeping apes as pets. Domestication led to a rapid developmental change in ape physiology and they grew in both size and intelligence. In short order however, keeping apes as pets evolved into keeping apes as indentured servants. Modern cities across the globe enslaved healthy apes of various species, particularly gorillas, chimpanzees and orangutans.