A boy of about nine has been bitten by his mad dog, and badly injured. He has a seizure at the hospital and his nurse mother is wracked with worry. She rushes to his side when he he wakes up, only to find him sucking away at the plasma bag that was feeding into his arm.

Wow, you think to yourself. “Never seen THAT before.”

“Blood” is a sturdy, well-cast and superbly-acted thriller, a “Nurse Jackie” tale with vampiric leanings. It’ may be a genre picture, but it’s smart enough to send one to your favorite search engine to see what might explain what has infected little Owen, ably played by Finlay Wojtak-Hissong.

That’s what newly-divorced Nurse Jessica (Michelle Monaghan) does. She’s moved Owen and older sister Tyler (Skylar Morgan Jones) out to a farmhouse she spent much of her childhood in, and within a week, her son’s in the hospital and she’s doing online consulting with physicians to see what’s ailing him.

Because Jessica’s panic-stricken, and Jessica’s noticed something. What might her son’s seeming preference for “oral transfusions” indicate, Doctor?

Jessica can’t talk about this with anyone else. She can’t let Owen go to school, because as she figured out in the hospital, only one thing keeps his fever-spiking and flatlining at bay — “Blood.” Letting him decide where he gets it would be a mistake.

Skeet Ulrich plays the ex who took up with, impregnated and married the nanny — we’re not sure in which order. He wants a bigger share of custody, and that’s when this black and white situation turns grey.

Jessica’s hours and the nature of her work gave her the stress and access to start using drugs. And here she is, fighting for her kids with that ex, trying not to look or sound crazy or drugged, struggling to keep her little boy alive by stealing plasma, and knowing she and Owen “can’t tell ANYone” what’s going on.

Just spitballing here, but I’d say “custody” is very much up in the air here.

Will Honley’s script — he did “Bloodline,” so he’s found his niche — teases out who learns about this Type A predicament, and when. I really like the depiction of Owen as a headstrong, impulsive kid in early scenes, which makes “controlling” him and his cravings all the more precarious.

And you thought making your kid take care of her BRACES was tough.

Director Brad Anderson did “The Machinest,” an indie triumph for Christian Bale almost 20 years ago, and he takes his time with this, avoiding — probably to his detriment, considering the “horror” label — cheap scares and simple jolts.

“Blood” is a movie of family connections, motherly devotion and a dilemma that ranks right up there with “Sophie’s Choice.” It’s not overstating the case to say this has genuine heartbreak in it as Jessica faces one horrible choice after another, as a mother and a nurse.

Aside from a moment here and there in the Jamie Lee Curtis canon, I’m hard-pressed to think of another recent horror film that drew a tear and made the emotional connections “Blood” does.

It’s not high art or a great film, just a genre tale with a twist. And it’s a tad predictable, by the time that third act rolls around. But Monaghan and the kids sell the premise, and the movie plays.

Rating: unrated, bloody violence, profanity

Cast: Michelle Monaghan, Skylar Morgan Jones, Finlay Wojtak-Hissong, June B. Wilde and Skeet Ulrich

Credits: Directed by Brad Anderson, scripted by Will Honley. A Vertical release.

Running time: 1:49