If the average number of cases in some small area (e.g. a block or blocks of buildings, a part of town, a town) is e.g. two patients with symptoms X per MONTH, and the symptoms include FEVER and not being able to breath, i.e. something people can't just make up and can be easily measured, and THEN you get a patient from a block of buildings with symptoms X and THEN you test him and get a positive result and THEN you get ten more people from there with symptoms X, AND in the same small are some people have the same symptoms and are in such a bad condition that they need to be connected to the MACHINES and some DIE, you can be very sure it's a virus which never existed among the humans before generating these symptoms.

Otherwise, it would have to be some OTHER virus not known to humanity appearing at the same time, behaving unlike the viruses we know and watch every year, magically appearing exactly in the community where you verified the existence of the one for which you test.