STID had discussed terrorism and in an intriguing way, post-9/11, and how many times has Trek discussed pandemics to the same level it talks about other issues...?
Apart from a throwaway line of "Stardate 12345.6something, we are headed to Plot Device II in the Maudelin Cluster to pick up a vaccine so we can cure a disease because this is season one and vaccines are used to prevent diseases and not cure them", then someone gets kidnapped, Picard throws a tantrum and uses half a dozen photon torpedoes to detonate around the planet's orbit to terrify everyone with even though they're completely innocent, never mind he just broke among other things his own Prime Directive he uses as means to let people die in a few dozen other stories because it's the galactic plan Sarjenka knows nothing about or in that overly-contrived masterpiece society or a dozen other episodes...
So it can be done, absolutely. Especially when TOS covered topical issues at the time said issues occurred (e.g. "A Private Little War" hammered in somewhat tactlessly a war of significance happening at the time, even using a name callout... so why not for any virus?)
They don't even need to use another race or evil admiral o' t' week cliches. Leftover weapons from another civilization discovered and brought back to Planet Plotdevice IV and oops the technician slipped and the safety seals in the room malfunction - there, it's a new plot. Okay, it's not really new but so underused that after all this time that it feels fresh and new. Or better yet, the thing went boom in the sealed room and for whatever happenstance the room seals failed (solar flares far greater than expected* based on all known models and projections, it's a trinary star system too, why not) and not due to evil admiral moohahahaha subplot.
* in which case, if solar flares and EMP and all that are so risky, why would anyone build there? Why do people build tunnels right under lakes, seas, or oceans? Or build insanely tall buildings and/or nuclear reactors near earthquake zones? Same reasons could be applied to this solar system and a greater good.