There needs to be a little loathing, a dash of disgust, a river of revulsion to make a rivalry rise out of nowhere. A sprinkle of odium is helpful. A pinch of abhorrence. Maybe a couple of spoonfuls of outright hatred.

You throw that combustible mix in a blender …

And voila!

You have Yankees versus Astros.

There is no natural reason why there should be such ferocity between these two teams. They play in ballparks that sit 1,637 miles away from each other. They play in different divisions. For the first 51 years of the Astros’ existence they played in different leagues. The Astros and the Mets (who were born in the same year, 1962) had a thing once, briefly, because of a heated meeting in the ’86 NLCS. But that passed in a minute.

So there was no way to see this one coming, which is part of what makes it so great. Most of the rivalries we develop in this town happen that way. They are different, in their way, than the geographic rivals — Rangers/Islanders, Giants/Eagles, Knicks/Celtics, even Yankees/Red Sox. Those are almost logical. They’re awesome in their own way, but they make sense.

Yankees-Astros?

As recently as 2014, if you’d asked a Yankees fan what they felt about the Astros, the hunch here is that 95 percent would look at you and say, “I don’t feel anything about the Astros.” The Astros once threw a six-pitcher no-hitter at the Yankees, but that’s not exactly enough to fuel a furnace.

Now?

Even casual Yankees fans get red-faced at the mere mention of the Astros. They get angry. They believe they were robbed, wronged, ripped off by the Astros in 2017, and even if the official verdict was that by 2019 the ’Stros were playing mostly on the up-and-up … well, no Yankees fan believes that. They’ve all studied the replays of Jose Altuve’s series-clinching homer and his reaction in the home-plate dogpile more often than the Warren Commission replayed the Zapruder Film.

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“These are two excellent teams and when we play each other great things are going to happen on the field,” is the way Aaron Boone diplomatically put it during the summer, and that part of the equation is also true, and a necessary part of bumping this to where it is. But the Astros aren’t just good, the absolute equals of the Yankees. They make for easy villains, too. And that’s how your blood-and-guts rivals are born.

It’s how Patriots-Giants became a thing, thanks both to two meetings in the Super Bowl and the fact that the Giants won both times, once preventing the Pats from finishing off a perfect 19-0 season. Add to that the fact that the Pats were already living under the cloud of suspicion — and worse — after being caught using illegal taping practices earlier that year. For years, the Giants were actually Boston’s team, before the Pats were even born. Not anymore.

It’s how Knicks-Heat became a thing, thanks to Pat Riley’s defection (in New York that’s still spelled “b-e-t-r-a-y-a-l”) after the 1995 season. As heated as Bulls-Knicks had been, it was never easy to paint Michael Jordan with a villainous brush, and much as Knicks fans wanted to hate Phil Jackson the fact is he was an essential part of the team’s most glorious time.

But Riley? That first night back at the Garden, December 1995, there was some thought that some of the bile might turn visceral. Riley gleefully egged it on by waving his arms when he was introduced to a cascade of boos. And then the teams went on to become perennial playoff foes that sometimes resulted in on-court fisticuffs (Jeff Van Gundy holding on to Alonzo Mourning’s leg will never not be hilarious) and always resulted in some of the bloodiest, beastliest basketball ever seen.

The Astros and the Yankees aren’t going to give us that — not likely anyway. But for as long as this ALCS lasts, Yankees fans will not just want their heroes to win four out of seven games, they want a pound of Astro flesh, too. They want blowouts, bludgeonings, hammerings and thumpings. That still won’t make up for 2017.

But it’ll be a start.