From Halopedia, the Halo wiki
Hydra is a UNSC general emergency code. It is almost never used due to its content. The Hydra code word signifies imminent threat from biological weapons of mass destruction.
In 2552, Cortana declared code Hydra via a Forerunner slipspace carrier wave after the discovery of Installation 05 and its escaped Flood. The threat came from both the potential for the Halo's activation and the loose Flood that posed a significant threat to all life.
In 2553, cryo-jars risked turning into a "Code Hydra problem".