theoretical virus
A Theoretical Virus is a fictional virus that is based on a real virus, but does not actually exist. They are used as a benchmark for viral preparedness. A good example of a theoretical virus is lyssa x, which is a relative of rabies. Web groups such as OZORT are geared toward preparing for worst case scenario viruses like these.
A Theoretical Virus can be just as dangerous as a real virus if you aren't prepared for it.