The monkeys were either from zoos or from the wild and they were used by Dr. Warren and Dr. Clive as non-human animal test subjects because their first test subject (a violent human criminal) had gone disastrously wrong. The doctors injected chimpanzees with the enhancer drug, which suddenly mutated, due to the already high aggressive qualities of the chimpanzees. This made the monkeys extremely violent and aggressive.
The monkeys became the first infected organisms, and they caused chaos in their cages, screaming and thrashing and drooling blood. The monkeys wanted to be released to kill or Infect other humans. These monkeys seemed much more eager for violence than people and their eyes glowed red.
Role in 28 Days Later
The monkeys were visited by an animal rights group, who were informed by the late Dr. Warren of the experiment and how bad it was. The group didn't know of the virus they only believed the scientists were evil and abusing animals. They were soon accosted by Dr. Clive who was scared but he tried to tell them of the virus. The activists didn't listen, and, determined to open the cages, they released a screaming monkey out who bit a woman. This started the Rage virus outbreak. The monkey who escaped was killed by Dr. Clive, but others broke out and escaped into Cambridge city, according to the comics.