Why Are Scientists Developing A Vaccine For A Virus That Doesn't Yet Exist?

UK scientists have begun developing vaccines as an insurance against a new pandemic caused by an unknown "Disease X". The work is being carried out at the government's high-security Porton Down laboratory complex in Wiltshire by a team of more than 200 scientists. Question: how does a team of scientists develop a vaccine if they don't yet know what they are creating a vaccine for? Perhaps they already know what it is and when it will be 'leaked' from a lab in 2024?