The cause of Donald Trump’s erratic behavior has been the subject of rampant speculation for years.

Some have pointed to the hair-loss drug Trump has admitted to taking, others have pointed to a 1992 article in Spy magazine which alleged that Trump was prescribed amphetamines as diet pills by a notorious Manhattan doctor.

This week, two women have come forward with allegations that Trump had unprotected sex with them while he was married. Ex-Playboy model Karen McDougal and porn star Stormy Daniels have both said Trump had unprotected sex with them during the same weekend in Lake Tahoe.

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That has led to chatter about an old theory that Trump’s behavior may be tied to untreated syphilis.

They’ll get Trump the same way they got Capone. That’s right: syphilis. — MKupperman (@MKupperman) March 13, 2018

So I was thinking….given Trumps bizarre behavior, his inability to complete a thought, his ease of getting lost in a simple sentence……and his propensity to have unprotected six with women who have had many partners….ya think it might be syphilis? Tertiary? — Madeline Peters (@ladyfromirvine) March 25, 2018

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Mr. President, Please arrange a time to meet with me. I believe you may have contracted syphilis from your days in “Vietnam”. Neurosyphilis explains many of your symptoms: sudden changes in personality & mental stability, confusion, memory problems, & poor concentration -House — Greg House, MD (@DrHouseResists) March 8, 2018

Anyone else think Trump has undiagnosed syphilis? — Samantha Sanderson (@SamSanderson123) March 24, 2018

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This is not a new theory. In a February 2017 article for The New Republic, a California doctor posited that Trump may have untreated late-stage neurosyphilis.

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“Commonly recognized symptoms include irritability, loss of ability to concentrate, delusional thinking, and grandiosity. Memory, insight, and judgment can become impaired. Insomnia may occur. Visual problems may develop, including the inability of pupils to react to the light. This, along other ocular pathology, can result in photophobia, dimming of vision, and squinting. All of these things have been observed in Trump.”

A Swedish doctor joked about the same thing on Twitter. Comedian Chelsea Handler went to investigate the theory.

Trump has publicly discussed this possibility given his lifestyle. In 1997, Trump joked that avoiding STDs was “my personal Vietnam.”

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“It is a dangerous world out there—it’s scary, like Vietnam. Sort of like the Vietnam era. It is my personal Vietnam. I feel like a great and very brave solider.”

Other politicians who have been theorized to have untreated neurosyphilis include Hitler , Ugandan cannibal Idi Amin and Mussolini.

It’s worth pointing out that the stigma around STDs is silly and dangerous, and that if Trump has syphilis he could be cheaply and efficiently treated with Penicillin.