Al Franken recently raised a provocative question about Donald Trump: Is he mentally ill? On HBO’s Real Time with Bill Maher last week, the Minnesota senator claimed that some of his Republican colleagues have “great concern about the president’s temperament,” adding that “there’s a range in what they’ll say, and some will say that he’s not right mentally. And some are harsher.” Two days later, he told CNN’s Jake Tapper, “We all have this suspicion that—you know, that he’s not—he lies a lot…And, you know, that is not the norm for a president of the United States, or, actually, for a human being.”

Franken is not alone in his suspicions. Last year, Jeb Bush said of his Republican primary opponent, “I’m not a psychiatrist or a psychologist, but the guy needs therapy.” Senator Bernie Sanders recently called Trump “delusional in many respects, a pathological liar.” And Congressman Ted Lieu, a California Democrat, is introducing legislation that would require the White House to have a psychiatrist on staff. “I’m looking at it from the perspective of, if there are questions about the mental health of the president of the United States, what may be the best way to get the president treatment?” he told the Huffington Post.

Meanwhile, a debate is raging among mental health professionals about Trump’s mental state, and whether it’s unethical of them to speculate publicly about someone whom they haven’t examined (doing so violates psychiatrists’ code of ethics, the relevant section of which is called “the Goldwater rule” because of its association with a magazine survey of psychiatrists about Barry Goldwater, the Republican presidential nominee, in 1964). One online petition with nearly 24,000 signatures calls for Trump’s removal because he “manifests a serious mental illness that renders him psychologically incapable of competently discharging the duties of President.” Another with 36,000 signatures declares that “Trump appears unable to control his compulsion and displays characteristics of all nine criteria to officially diagnose an individual with Narcissistic Personality Disorder.”

Physicians like me have also taken notice of Trump’s bizarre, volatile behavior. Given our experience, we can’t help but wonder if there’s a medical diagnosis to be made. After all, many medical conditions exhibit their first symptoms in the form of psychiatric issues and personality changes. One condition in particular is notable for doing so: Neurosyphilis.

Syphilis, a sexually transmitted infection, is sometimes referred to as “The Great Imposter” because of its ability to mimic many other conditions. It is commonly broken down into three stages. Primary syphilis is the most widely recognized form of the disease. It is characterized by the development of an ulcer, usually genital, a few weeks to a few months after sexual contact with an infected person. If the ulcer is not noticed, or not treated, it heals on its own, and the disease enters a dormant phase. But during this time, the bacteria—a spirochete called Treponema pallidum—spreads throughout the body without causing any symptoms.