Polyomaviruses are a family of small, hardy viruses that infect the vast majority of people worldwide, usually without causing any symptoms at all. Between 80% and 90% of the general adult population carries at least one type of polyomavirus, typically picked up during childhood. These viruses settle quietly into the body and persist for life, only becoming dangerous when the immune system is severely weakened.
How Polyomaviruses Spread and Take Hold
Most people acquire their first polyomavirus infections before 18 months of age. The two best-known types, BK virus and JC virus (both discovered in 1971 and named after the initials of the patients they were found in), are believed to spread through respiratory droplets or a fecal-oral route, much like a common cold or stomach bug. Other types, like Merkel cell polyomavirus, are a natural part of skin flora and likely spread through direct skin contact or environmental exposure. Some polyomaviruses have been detected in respiratory and stool samples from young children, though pinning down the exact route for each type has been difficult because the viruses are so widespread.
Once inside the body, the virus enters cells by binding to sugar molecules on cell surfaces. It gets taken up into tiny bubble-like compartments and eventually makes its way to the cell’s nucleus, where it can either replicate or go dormant. For the vast majority of people, the immune system keeps the virus in check indefinitely. BK virus tends to settle in kidney and urinary tract cells, while JC virus takes up residence in the kidneys and, critically, in certain brain cells.
When Polyomaviruses Become Dangerous
Polyomavirus-related diseases are almost exclusively problems of immune suppression. Organ transplant recipients taking drugs to prevent rejection, people with advanced HIV, and patients on certain immune-modifying medications are the ones at real risk. In these individuals, the virus can reactivate from its dormant state and begin replicating unchecked.
The specific disease depends on which virus reactivates and where it lives in the body.
BK Virus: Kidney and Bladder Problems
BK virus is the most common polyomavirus problem in transplant medicine. In kidney transplant recipients, uncontrolled BK replication in the transplanted kidney can lead to a condition called polyomavirus-associated nephropathy, which can damage or even destroy the graft. In bone marrow transplant recipients, BK virus more often attacks the bladder lining, causing hemorrhagic cystitis: painful, bloody urination that ranges from mild (blood only visible under a microscope) to severe (visible blood clots and kidney damage from urinary obstruction).
Symptoms fall into two patterns. The irritative pattern includes burning or painful urination and pelvic pain. The obstructive pattern involves blockage from bladder inflammation, which can back up and damage the kidneys. About two-thirds of patients who develop BK-related disease show high levels of the virus in their blood before symptoms appear, which is why transplant centers routinely screen for BK virus after transplantation. Viral loads above 10,000 copies per milliliter in the blood or above 10 million copies per milliliter in urine are the thresholds that typically prompt intervention.
JC Virus and Brain Infection
JC virus causes the most feared polyomavirus complication: progressive multifocal leukoencephalopathy, or PML. This is a brain infection where the virus destroys the cells that produce myelin, the insulating coating around nerve fibers. Without that coating, nerve signals break down, and the resulting damage can be devastating.
PML typically develops in people with severely weakened immune systems, including those with advanced HIV or those taking powerful immune-suppressing medications like natalizumab (used for multiple sclerosis). Symptoms depend on which part of the brain is affected but commonly include weakness on one side of the body, difficulty walking, vision loss in part of the visual field, trouble speaking, and cognitive or behavioral changes. One-third to one-half of patients experience thinking and personality changes. Seizures, headaches, and double vision occur less frequently.
On brain MRI, PML appears as bright patches on certain scan types, often at the boundary between gray and white matter. The frontal lobes and the back of the brain are most commonly involved, though lesions can appear almost anywhere. Unlike multiple sclerosis lesions, which tend to be small and clustered near the brain’s fluid-filled spaces, PML lesions are often larger and located closer to the brain’s surface. There is no reliable antiviral treatment for PML. The primary strategy is restoring immune function, whether by starting HIV treatment or stopping the immune-suppressing medication that allowed the virus to reactivate.
Merkel Cell Polyomavirus and Skin Cancer
Merkel cell polyomavirus stands apart from its relatives because of its link to cancer. This virus is the cause of Merkel cell carcinoma, a rare but aggressive skin cancer. Most people carry the virus harmlessly on their skin. Cancer develops only when the virus’s DNA accidentally integrates into a host cell’s genome and then acquires specific mutations that delete part of the viral code.
The key players are two viral proteins called large T antigen and small T antigen. In normal infection, large T antigen helps the virus copy itself. But in Merkel cell carcinoma, the end of the gene encoding this protein is always cut short by mutations. This truncation eliminates the virus’s ability to replicate (which would kill the cell) while preserving its ability to override the cell’s built-in brakes on growth. Specifically, the protein disables a critical tumor suppressor called Rb, forcing cells into a state of constant division. It also boosts levels of a protein called survivin that prevents the cell from self-destructing.
The small T antigen contributes by stabilizing several proteins that drive cell growth, including well-known cancer-promoting factors. Together, these two viral proteins hijack normal cell controls and push infected cells toward uncontrolled growth. Merkel cell carcinoma most often appears in older adults and people with compromised immune systems, typically as a painless, rapidly growing nodule on sun-exposed skin.
How Polyomavirus Infections Are Monitored
Because most polyomavirus infections cause no symptoms, there’s no reason to test the general population. Monitoring matters mainly for transplant recipients and others on long-term immune suppression. The standard tool is quantitative PCR, a lab test that measures the exact amount of viral DNA in blood or urine samples. Transplant centers typically check BK viral loads at regular intervals after surgery, often monthly for the first year.
The goal is to catch rising viral levels before they cause organ damage. When blood levels of BK virus climb above 1,000 copies per milliliter and stay there, or cross 10,000 copies per milliliter, guidelines recommend reducing immune-suppressing medications in a stepwise fashion. This gives the patient’s own immune system a better chance of bringing the virus back under control while trying to avoid organ rejection.
Treatment Options
There is no antiviral drug proven to work against polyomaviruses. The cornerstone of management is reducing immunosuppression to allow the body’s own defenses to suppress the virus. For BK virus in kidney transplant recipients, this means carefully dialing back anti-rejection medications according to a predefined schedule. International consensus guidelines from 2024 specifically note that commonly tried drugs like cidofovir, leflunomide, and quinolone antibiotics have not shown reliable benefit and are not recommended outside of clinical trials.
One promising approach is virus-specific T-cell therapy. In this treatment, immune cells that specifically recognize BK virus are grown in a lab, either from the patient’s own stem cell donor or from unrelated donors, and then infused into the patient. In a clinical trial of 41 transplant recipients treated between 2017 and 2019, the overall response rate was 86% for patients with BK virus in the blood, and 100% for those with hemorrhagic cystitis. No significant side effects from the infusions were reported. This therapy is still considered experimental, but the results are encouraging enough that guidelines recommend enrolling eligible patients in clinical trials when standard approaches fail.
For PML caused by JC virus, restoring immune function remains the only effective strategy. In HIV patients, this means initiating or optimizing antiretroviral therapy. In patients on immune-suppressing medications, it means stopping the offending drug. Even with immune recovery, PML can leave lasting neurological damage, and outcomes depend heavily on how early the condition is caught.