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.

A secondary stage of the disease is seen in some patients weeks or months later. These patients may develop a variety of systemic symptoms, such as rash, fever, and swollen glands. If not treated, the infection enters a prolonged latent phase, which can last decades. During this time, it is asymptomatic and it is not contagious. In some cases, this is followed by a tertiary stage, which is the most serious and may involve any organ in the body. It is seen 10 to 30 years after the initial infection, and is best known for causing neurologic and neuropsychiatric disease: Neurosyphilis.

The symptoms of neurosyphilis are protean, varying widely from one individual to another. 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. Dementia, headaches, gait disturbances. and patchy hair loss can also be seen in later stages of syphilis.

Does Trump suffer from this condition? I cannot, of course, establish this diagnosis from a distance. There’s a great deal of information I don’t have access to, which could be critical in reaching the correct conclusion. In Trump’s case, there are many diagnostic possibilities, and we have very little background information because the slim medical summary he released was vague, unverifiable, and possibly outdated.

On the other hand, every time I see a new patient, he or she comes to me with incomplete information, or sometimes no information at all. Part of my training is to ask the right questions to get a sense of what the problem might be and make a list of possible diagnoses that could explain the problem. This is called the “differential diagnosis.” From there, additional questions, examinations, and tests are performed that narrow down the list, usually to one unifying diagnosis.

Trump poses with Miss America contestants on his yacht in Atlantic City, New Jersey, in 1988.JACK KANTHAL/ASSOCIATED PRESS

Given the limited information available, neurosyphilis belongs in the differential diagnosis. We know Trump was potentially exposed to syphilis based on his own statements that he was sexually promiscuous in the 1980s, a period when syphilis cases were rapidly increasing in the U.S. “I’ve been so lucky in terms of that whole world,” he told Howard Stern in 1997, referring to his dating life the decade prior. “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.”

If indeed Trump has neurosyphilis, he’d be in famous company. Al Capone had it. So did composers Frederick Delius and Franz Schubert. Many others were suspected of having it, including Hitler, Mussolini, and Ivan the Terrible. What might enable us to eliminate Trump from this group? Two simple blood tests, in combination, can determine whether a patient has syphilis now or had it in the past. If both tests are negative, then he doesn’t have neurosyphilis. If one or both tests are positive, further evaluation, probably including a spinal tap, would be in order.

The importance—both to Trump and the nation—of establishing or ruling out this diagnosis cannot be overstated, because this infection is treatable. Without treatment, however, the disease is progressive: It can make for a rather ugly end to one’s life. Whether Trump emerged unscathed from his “personal Vietnam” is now a question that he should explore with his personal physician.