Syphilis, caused by the corkscrew-shaped bacterium Treponema pallidum, works through the body in stages, and what it does depends entirely on how long it goes untreated. In the first weeks it produces a painless sore. Within months it can scatter a rash across the skin and inflame organs. Left alone for years, it destroys blood vessels, eats holes in tissue, and invades the brain, where it can cause dementia, paralysis, and death. The bacterium is remarkably fragile outside the human body yet remarkably tenacious inside it, surviving for decades by dodging the immune system in ways researchers are still working to fully understand.
How the Infection Begins
T. pallidum enters the body through mucous membranes or tiny breaks in the skin during sexual contact. Despite having an unusually small genome and lacking many of the toxins and metabolic tools that other dangerous bacteria rely on, it is highly infectious and can persist in an untreated host for decades.1Europe PMC. Biological basis for syphilis Once inside, it slips into the bloodstream quickly, spreading far beyond the original site of contact within days. That rapid dissemination is what makes syphilis a whole-body disease rather than a localized infection, and it explains why the bacterium can show up in the brain, the heart, and the bones even though initial contact was limited to the genitals or mouth.
Primary Syphilis and the Chancre
The first visible sign of infection is usually a chancre: a firm, round sore that appears at the spot where the bacterium entered, typically three to six weeks after exposure. The chancre is painless, which is part of what makes early syphilis easy to miss. Under a microscope, the sore shows dense inflammation. Treponemes cluster throughout the deeper skin layers and around blood vessels, surrounded by plasma cells and lymphocytes that the immune system has rushed to the scene.2PubMed. Primary and secondary syphilis: a histopathological study
What happens next at the chancre site matters for the long-term course of the disease. A strong immune response at this stage can clear many of the organisms locally, and the chancre heals on its own within a few weeks. But the quality of that immune response varies from person to person. When the body mounts the wrong type of defense, the infection is more likely to persist and eventually progress to its later, more destructive stages.3PubMed Central. The immunopathobiology of syphilis: the manifestations and course of syphilis are determined by the level of delayed-type hypersensitivity Because the chancre heals whether or not the underlying infection has been controlled, people often assume they are fine, which is one reason syphilis so frequently goes untreated past its earliest and most curable stage.
Secondary Syphilis and Systemic Spread
If the infection is not caught during the chancre phase, it enters a secondary stage weeks to months later, and this is where the bacterium announces that it has gone everywhere. The hallmark is a rash, often on the palms of the hands and soles of the feet, locations unusual enough to serve as a diagnostic clue. But the skin findings are only the most visible sign. Secondary syphilis can produce fever, swollen lymph nodes, sore throat, patchy hair loss, and lesions on mucous membranes. The disease has historically been called “the great mimicker” because it can look like dozens of other conditions.4Europe PMC. Do not miss secondary syphilis: examine the palms and soles
During this stage, the moist, flat lesions known as condylomata lata that can appear around the genitals and anus are among the most infectious manifestations of the disease. A person in secondary syphilis who does not know their diagnosis can transmit the bacterium very easily. Even after the rash and other symptoms resolve, which they do on their own, the infection has not gone away. It has simply gone underground.
The Quiet Years of Latent Syphilis
After secondary symptoms fade, syphilis enters a latent phase in which there are no outward signs or symptoms at all. The spirochaete persists in the body during this period, and the person can still be infectious, particularly in the first year or so.5Nature Reviews Disease Primers. Syphilis This silent interval can last years or even decades. Some people remain in latent syphilis for the rest of their lives without ever developing further problems. Others progress to tertiary disease, where the real destruction begins. There is no reliable way to predict which path a given person will take, which is why treatment at any stage is strongly recommended.
How Syphilis Gets Into the Brain
One of the more unsettling facts about syphilis is how early the bacterium reaches the central nervous system. T. pallidum can invade the brain and spinal cord within days of the initial infection, during or even before the chancre appears. Studies of spinal fluid in people with early syphilis have found evidence of neuroinvasion in up to about 30 percent of cases.6Clinical Infectious Diseases. State-of-the-Art Review: Neurosyphilis Most of the time, the immune system clears this early invasion without lasting consequences. But in a minority of people, the organisms take hold in the nervous system and set the stage for symptomatic neurosyphilis later on.7PubMed. Neurosyphilis
The early forms of neurosyphilis include inflammation of the membranes surrounding the brain (meningitis), which can cause headache, stiff neck, and sensitivity to light. Meningovascular syphilis takes things further: the bacterium inflames blood vessels inside the brain, which can narrow or block them entirely. When a blood vessel in the brain becomes occluded, the result is a stroke. Case reports have documented fatal strokes from syphilitic vasculitis affecting arteries at the base of the brain, with pathology showing inflamed, clotted vessels.8The American Journal of the Medical Sciences. Meningovascular Syphilis With Fatal Vertebrobasilar Occlusion In a young person with no typical stroke risk factors, syphilis should be on the list of possible causes.
What Late Neurosyphilis Does to the Mind and Body
If neurosyphilis goes untreated for years, it can progress into two particularly devastating forms. The first is general paresis, sometimes called “general paresis of the insane” in older medical literature. This is syphilitic dementia. It was once one of the most common causes of admission to psychiatric institutions, typically striking relatively young men. The illness was characterized by behavioral changes, sometimes with grandiose delusions, followed by progressive cognitive decline, difficulty walking, and eventually paralysis and death.9Oxford Academic (Brain). General paresis of the insane: a diagnosis not to forget
Modern studies of patients with general paresis confirm that cognitive impairment spans multiple domains. Memory, language, spatial reasoning, and executive function all decline, and the pattern resembles the kind of broad cognitive erosion seen in other forms of dementia.10PubMed Central. Neuropsychological Features in Patients with General Paresis of the Insane at an Early Stage Because syphilis is treatable and the dementia it causes can sometimes be slowed or partially reversed with antibiotics, clinicians are urged not to forget it as a diagnosis even today. A person presenting with personality changes and cognitive decline who tests positive for syphilis may have a curable form of dementia, a rarity in neurology.
The second late form is tabes dorsalis, which targets the spinal cord rather than the brain. Slow, progressive degeneration of nerve cells in the spinal cord leads to a triad of symptoms: a staggering, unsteady gait; sudden, shooting “lightning” pains in the legs or torso; and loss of bladder control.11PubMed Central. Magnetic resonance imaging of the spinal cord in a man with tabes dorsalis Patients also lose the ability to sense where their limbs are in space, making it hard to walk in the dark. Tabes dorsalis was far more common before penicillin became available, but it still appears in people with undiagnosed or inadequately treated syphilis.
Eyes and Ears at Risk
Syphilis does not limit its neurological damage to the brain and spinal cord. Ocular syphilis and otic syphilis can occur at any stage of the disease, not just during late neurosyphilis. Ocular involvement often shows up as uveitis, an inflammation inside the eye that causes redness, pain, light sensitivity, and blurred vision. If untreated, it can lead to permanent vision loss. Otic syphilis typically manifests as sudden sensorineural hearing loss, sometimes accompanied by ringing in the ears or dizziness. Case reports describe patients presenting with both eye and ear involvement simultaneously.12PubMed Central. All Eyes and Ears: A Case of Neurosyphilis Presenting With Simultaneous Ocular and Otic Involvement Because these symptoms can appear early, anyone with unexplained vision or hearing changes and risk factors for sexually transmitted infections should be tested for syphilis.
How Syphilis Damages the Heart and Blood Vessels
Cardiovascular syphilis is a tertiary complication that typically takes ten to thirty years of untreated infection to develop. The primary target is the aorta, the largest artery in the body. T. pallidum infects the tiny blood vessels that supply the aortic wall itself, a process called obliterative endarteritis of the vasa vasorum. As those supply vessels are destroyed, the aortic wall weakens from ischemic injury, losing its elasticity and structural integrity. The result can be an aneurysm, a dangerous ballooning of the aorta that risks rupturing.13Cardiology in Review. Syphilitic Cardiac and Vascular Disease: A Comprehensive Review
The damage does not stop at the aortic wall. The aortic valve can become incompetent, leaking blood backward with each heartbeat. The openings where the coronary arteries branch off from the aorta can narrow, reducing blood flow to the heart muscle and causing angina or heart attacks. Syphilitic aortitis was once a leading cause of aortic aneurysm. It is rarer today but still occurs in populations where syphilis goes undiagnosed for years.
Gummas and Tissue Destruction
Gummas are the signature lesion of tertiary syphilis, and they can appear almost anywhere in the body: the skin, bones, liver, and brain are the most common sites. A gumma is essentially a mass of chronic inflammation centered on a core of dead, cheese-like tissue. Its formation appears to be driven by a delayed immune overreaction to the spirochaete. As inflammatory cells flood in and small blood vessels become inflamed and blocked, the local tissue loses its blood supply and dies.14Frontiers in Medicine. Case report: Cerebral syphilitic gumma: a case retrospective report of eight cases
Gummas in the skin can ulcerate and leave scars. In the bones, they cause pain and structural weakness. In the brain, a syphilitic gumma can mimic a tumor, appearing as a mass on imaging and sometimes requiring biopsy before the correct diagnosis is made. The irony of the gumma is that it represents the immune system trying to fight the infection but causing collateral damage in the process. With antibiotic treatment, gummas can resolve, sometimes dramatically, but the scarring and tissue loss they leave behind may be permanent.
How the Bacterium Stays Hidden
The fact that T. pallidum can survive in a human host for decades despite a functioning immune system is one of the more remarkable things about this pathogen. The bacterium has long been called a “stealth pathogen” because its outer surface is unusually bare, lacking the kinds of proteins that normally alert the immune system to an invader. On top of that, there is growing evidence that T. pallidum uses antigenic variation, essentially changing the few surface molecules it does display, to stay a step ahead of immune defenses.15PubMed Central. Treponema pallidum, the stealth pathogen, changes, but how?
This evasion is the reason syphilis has a latent phase at all. The immune system is aware of the infection (antibody tests remain positive), but it cannot finish the job. The spirochaetes persist in low numbers, probably sheltered in tissues where immune surveillance is limited, waiting out the years until, in some people, they re-emerge to cause tertiary disease. It is also why reinfection is possible: recovering from syphilis does not produce lasting immunity the way measles does.
When a Pregnant Person Has Syphilis
Congenital syphilis occurs when T. pallidum crosses the placenta and infects the fetus. Transmission can happen at any point during pregnancy.16Radiology Case Reports. Bone abnormalities in congenital syphilis: A case report The consequences can be severe: stillbirth, premature delivery, low birth weight, and a range of organ damage in the newborn. Among infants born alive with congenital syphilis, complications arise from persistent inflammation and scarring caused by the infection in various organs, including the bones, liver, and nervous system.17PubMed Central. Skeletal manifestations of congenital syphilis: Rare but clinically relevant
Skeletal involvement is a particularly telling sign. Bone abnormalities visible on X-ray can be an early clue to the diagnosis in a newborn who otherwise looks healthy. Later in childhood, untreated congenital syphilis can cause distinctive dental malformations, hearing loss, and joint inflammation. Congenital syphilis is almost entirely preventable with routine prenatal screening and timely treatment of infected mothers, which is what makes the sharp rise in cases in recent years especially frustrating for public health officials.
The Syphilis-HIV Relationship
Syphilis and HIV have a well-documented synergy that works in both directions. The open sores and mucosal damage caused by syphilis create easy entry points for HIV. Meanwhile, the immune suppression caused by HIV may facilitate syphilis transmission and alter the typical course of syphilis infection, making diagnosis more difficult.18Russian Open Medical Journal. Bidirectional Association within HIV and Syphilis Coinfection: A Pathophysiological and Clinical Review People living with both infections may experience atypical presentations of syphilis, faster progression, or a poorer response to standard treatment. Research in sub-Saharan Africa has also suggested an association between syphilis history and difficulty achieving full viral suppression of HIV, though results have not always reached statistical significance.19PubMed Central. The Association Between Syphilis Infection and HIV Acquisition and HIV Disease Progression in Sub-Saharan Africa
From a practical standpoint, anyone diagnosed with syphilis should be tested for HIV, and vice versa. The two infections travel in the same populations and amplify each other’s impact. Clinicians managing coinfected patients typically monitor more closely and may need to adjust follow-up schedules, since standard serologic markers for treatment success can behave differently in HIV-positive individuals.
How Neurosyphilis Is Diagnosed
Diagnosing neurosyphilis is not as straightforward as diagnosing syphilis in the blood. Standard blood tests can confirm exposure to T. pallidum, but determining whether the bacterium has invaded the nervous system usually requires a lumbar puncture to examine the cerebrospinal fluid. The diagnosis rests on a combination of clinical signs and spinal fluid findings: elevated white blood cell counts, elevated protein levels, and positive results on both treponemal and nontreponemal tests performed on the fluid itself.20Frontiers in Public Health. Advances in laboratory diagnostic methods for cerebrospinal fluid testing for neurosyphilis
No single test is perfect. In one study evaluating a point-of-care diagnostic platform, treponemal testing of spinal fluid reached a sensitivity of about 86 percent and a specificity of about 86 percent, while nontreponemal testing reached a sensitivity of 80 percent and specificity of 97 percent.21PubMed Central. A Dual Platform Point of Care Test for Neurosyphilis Diagnosis Those numbers mean that a negative spinal fluid test does not completely rule out neurosyphilis, especially in someone with suspicious symptoms. Clinicians sometimes treat based on the clinical picture even when laboratory results are ambiguous.
Treatment and the Herxheimer Reaction
Penicillin remains the cornerstone of syphilis treatment and has been since the 1940s. For neurosyphilis, intravenous penicillin is required to ensure adequate drug levels in the brain. For people allergic to penicillin, doxycycline is the main alternative. Macrolide antibiotics like azithromycin are not considered appropriate for syphilis treatment, in part because resistance to macrolides has emerged independently across multiple circulating lineages of T. pallidum worldwide.22Nature Communications. Genomic epidemiology of syphilis reveals independent emergence of macrolide resistance across multiple circulating lineages
One peculiarity of syphilis treatment is the Jarisch-Herxheimer reaction, a temporary flare of symptoms that occurs within hours of the first antibiotic dose. Patients often develop fever, chills, headache, and muscle aches, and any existing rash may temporarily worsen.23PubMed Central. Jarisch-Herxheimer Reaction After Cephalosporin Administration in Syphilis The reaction is thought to be triggered by the rapid die-off of spirochaetes and the release of bacterial components that provoke a surge of inflammatory signaling molecules. Experimental work points to spirochetal lipoproteins and a wave of phagocytosis as the initial triggers, followed by a burst of pro-inflammatory cytokines.24PubMed Central. The Jarisch-Herxheimer Reaction After Antibiotic Treatment of Spirochetal Infections: A Review of Recent Cases and Our Understanding of Pathogenesis Severe reactions are uncommon, and the episode usually resolves within 24 hours. It is, in a sense, the body’s inflammatory alarm going off as the infection is being destroyed.
An Ancient Origin Still Being Unraveled
Where syphilis came from has been debated for centuries. The “Columbian hypothesis” holds that European explorers brought the disease back from the Americas in the late 1400s, while the “pre-Columbian hypothesis” argues it already existed in Europe before contact. Recent genomic analysis of ancient treponemal DNA from pre- and peri-contact sites in the Americas has added new weight to the American-origin story. Researchers recovered five ancient treponemal genomes representing ancestors of the lineages responsible for syphilis, yaws, and bejel, and found that all characterized T. pallidum genomes, modern and ancient, trace back to an origin in the Americas that postdates human arrival there.25PubMed Central. Ancient genomes reveal a deep history of Treponema pallidum in the Americas
The picture is not fully settled, as previously unsampled ancient populations could still reshape the phylogenetic tree. But the genomic evidence currently points toward a deep and diverse evolutionary history of treponemal disease in the Americas, with venereal syphilis as we know it emerging from that broader family of closely related infections. The practical relevance is that these subspecies are genetically very similar to one another, which is both why cross-immunity between them is partial and why developing a vaccine has been so difficult. Decades of effort have yet to produce one, and T. pallidum still cannot be grown continuously in the laboratory, a basic research limitation that slows work on nearly every front.