https://www.gamespot.com/articles/rainbow-six-siege-outbreak-now-live-on-test-server/1100-6456885/

Rainbow Six Siege fans who are eager to try out the game's upcoming limited time co-op mode, Outbreak, can now do so, provided they own the game on PC. Following the launch of the new Chimera Operators on the technical test server, another update has arrived that introduces Outbreak.