1 Steve: Erm... Jane, I have to tell you something. We can't tell the public there's a Balrog on the loose.
2 Jane Goodall: Indeed! The public can't know the truth about these cryptozoological entities. They'd panic.
3 Steve: Crikey! No, I mean...
4 Jane Goodall: We'll make up some cover story about terrorists releasing flesh-eating viruses across the city or something.

As far as I know (and I'm sure I'll get e-mails from microbiologists if I'm wrong) there are no flesh-eating viruses. There are, however, what are popularly known as "flesh-eating bacteria", although strictly speaking they don't eat flesh, they merely cause it to necrotise (i.e. the cells within it to die).

I'll provide a Wikipedia link here, but be warned, if you are squeamish about wounds, you might not want to visit this page because of the photos on it: Necrotising fasciitis.

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2023-10-24 Rerun commentary: Coming up with cover stories is one of the most fun parts of dealing with weird disease, monster, or alien outbreaks. Especially if you want to make people evacuate from the danger area, without letting them know precisely what the true danger is. You get to come up with something that sounds even scarier!

The good old industrial nerve gas spill train accident is a classic ("Why, even my dog has a gas mask!").