I'm trying to identify a hard science fiction short story I read in a hard-copy book anthology (Update: actually ClarksWorld) roughly 5 years ago. My impression is that it was a relatively new story at that time (Update: The story was about ten years old at that time).
I think it has a male western main character. He works at tracking and preventing the spread of a disease through Earth's population, although in practice that ends up looking more like the role of an armed detective. The disease is sentient, and takes over the behavior of the infected. It is only recently arrived on Earth, carried back accidentally from one of the moons of Jupiter. (The moon is named in the story, and it's one of the well-known ones, but I can't recall which moon it is.)
There is a paragraph where he imagines how the disease must have waited on that moon, through geological time, for a space-faring species to happen across it, which it could infect.
In his sleep, the MC has a recurring nightmare, of falling towards Jupiter, and the welcome oblivion of its crushing depths. But every time, before he reaches Jupiter, against all the odds, he hits that relatively tiny moon instead, where the terrible disease is waiting for him.
I think there is a (final?) scene in which the MC ends up with a gun, chasing a girl who is infected by the disease. Her words are its words.
I enjoyed the noir atmosphere, with him chasing leads, practically and with competence, but suffused with a feeling of desperate dread and inevitability.
In my memory I often conflate it with the short story "Leviathan Wept" by Daniel Abraham. I read them at the same time - although not, I think, in the same anthology. They both have a no-nonsense style, and share a (final?) scene where the male MC converses with a younger girl who represents the other.