Robert Sapolsky is a professor of biology, neurology and neurosurgery at Stanford University. He is also the author of " Behave: The Biology of Humans at Our Best and Worst ." The opinions expressed in this commentary are his own.

(CNN) The other day, our President expressed a remarkable opinion. Democrats "want illegal immigrants, no matter how bad they may be, to pour into and infest our Country, like MS-13," he tweeted , referring to the international gang.

"Infest." Now that's an interesting word, and there are at least two groups of people who are particularly well positioned to appreciate its choice.

The first group is neuroscientists who study a part of the brain called the insula. In most mammals, the insula does something mundane but important: If an animal bites into or smells a piece of food that is spoiled, the insula rapidly activates, triggering reflexes such as spitting the food out, curling the upper lip against the nose, maybe even vomiting.

This is mighty useful in terms of preventing ingesting toxins. Things work the same way in humans; stick a doughty volunteer in a brain imager, give them something fetid to bite into or smell, and the insula immediately activates. And as a measure of our cognitive sophistication, we humans can even activate the insula when thinking about eating something repulsive

But now, study something more interesting than rancid food. Show someone a picture of a lynching, of bodies piled high in a concentration camp, of Klansmen marching; make someone reflect on something awful they once did; stab them in the back with a betrayal. There's a good chance that the insula will activate as well.

At some time, tens of thousands of years ago, humans evolved the notion that norms of right and wrong behaviors could be systematized into moral systems. And in the process, evolution tinkered and improvised, expanding the portfolio of the ancient insula such that in humans, it not only mediates gustatory and olfactory disgust, but moral disgust as well.

It's why something sufficiently morally disturbing can make us feel sick to our stomach, want to throw up, or be left with a bad taste in our mouths.

This can be great, in that the insula's involvement helps build up the visceral head of steam that can be needed to right a deep moral wrong. But this versatility of the insula carries a danger: The temptation to use moral disgust as a litmus test.

How do you decide if the way someone eats, prays or loves is wrong? Just ask whether it makes you feel disgusted. If it makes you puke, then you must rebuke. The problem, of course, is that moral disgust is a moving target, and one person's moral disgust is another's normal, loving lifestyle. Moreover, visceral disgust is a great stepping stone for more abstract disgust; once you decide that someone eats disgusting things, it's just a hop, skip and a jump to deciding they think and feel disgusting things as well.

We all differ as to the workings of the insula, and this helps explain more than just why only some people would think the milk tastes a little bit off. As a fascinating finding, social conservatives tend toward lower thresholds for disgust than liberals.

They're more likely to be unsettled by wearing someone else's (clean) clothes, sitting on a chair still warm from a previous occupant, or thinking of someone spitting into a glass of water and then drinking it; show them a disgusting picture (e.g., a wound teeming with maggots) and their autonomic nervous systems tend to lurch more than a liberal's would (and as an important control, this lower threshold is not found among economic or geopolitical conservatives).

Perhaps, most importantly, you can manipulate people's moral judgments by exploiting the insula. Prime subjects to think of the United States as a living entity (discuss, for instance, how the United States underwent a "growth spurt" after the Civil War), and then have them read about scary new infectious diseases, and they express more negative views about immigrants.

Stick heterosexual subjects in a room with some smelly garbage, and they express more negative views about gay men. Upon smelling something subliminally vile, the insula confuses tasks and searches for something in your social world to stick with a "that's disgusting" label. It's not so much that old canard that a conservative "is a liberal who has been mugged." A temporary conservative can be a liberal who has been smelling rotting fish.

Scholars of insula neurobiology would likely give special attention to the President's imagery of immigrants "infesting" our land. But more importantly, another group would have a special appreciation of this as well.

When the Nazis urged on their populace toward the final solution, their propaganda was of Jews as rats , the Holocaust as extermination of the disgusting vermin in Germany's basement. For contemporary European white supremacists, it's the image of Islam as a malignancy. For Southern slavers, it was Africans as subhumans. And when the Hutus of Rwanda triggered the 1994 genocide that killed 75% of the Tutsi tribe in under 100 days, their propaganda endlessly shrieked about the Tutsis as cockroaches

Every effective genocidal propagandist intuitively knows about the insula -- get things to the point where invoking "Them" activates the insula in your followers and you'll have people goose-stepping in no time.

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Thus, Trump's tweet prompts not just neurobiological musings, but a historical one as well. He may not intend to create an association between immigrants and disgust but, psychologically, his rhetoric does not inspire compassion or unity -- American values.

So, constituents, be very alarmed when a leader tries to make you think of other humans as vermin. It's enough to make you sick to your stomach.