HHV-6, or human herpesvirus 6, is an extremely common virus that infects nearly everyone in early childhood. More than 80 to 90% of adults worldwide carry antibodies against it. Most people encounter HHV-6 between 6 and 15 months of age, when it causes roseola, the familiar illness marked by a sudden high fever followed by a pink rash. After that initial infection, the virus never leaves. It establishes a lifelong, dormant presence in the body, typically without causing further problems unless the immune system becomes severely weakened.
Two Distinct Viruses Under One Name
What was originally called HHV-6 is actually two separate viruses: HHV-6A and HHV-6B. Their genetic material overlaps by about 90%, but key regions differ by more than 30%, and the two viruses behave quite differently in the body. HHV-6B is responsible for virtually all childhood roseola cases in the United States, United Kingdom, and Japan, accounting for 97 to 100% of primary infections. HHV-6A appears to be more common in parts of sub-Saharan Africa, where it has been identified as the predominant strain in young children with fever-related illness.
The biological differences go deeper than geography. HHV-6A is better at infecting nerve cells. It can replicate in neural stem cells, brain support cells called astrocytes, and the cells that produce the insulating coating around nerves. HHV-6B cannot replicate efficiently in these same cell types. HHV-6B, on the other hand, has developed a way to resist certain antiviral signals the immune system uses (called interferons), which may help explain why it spreads so successfully in young children.
How HHV-6 Spreads and Where It Hides
The virus spreads primarily through saliva. Infectious virus has been detected in the saliva of nearly all individuals tested, making casual contact between caregivers and infants the most common route of transmission. A parent, older sibling, or other close contact who was infected years ago can still shed the virus from their salivary glands without knowing it.
Once the initial infection resolves, HHV-6 settles into a dormant state inside certain immune cells (monocytes and macrophages) and within the salivary glands. It can also persist in brain tissue. HHV-6 has a trick that is unique among human herpesviruses: it can splice its own DNA directly into the ends of human chromosomes, in regions called telomeres. This chromosomal integration happens in roughly 1% of the general population, meaning the viral DNA is copied every time a cell divides and can even be passed from parent to child through egg or sperm cells.
Roseola: The Childhood Illness
For most people, the only encounter with HHV-6 that matters is roseola, also called sixth disease. It typically strikes between 6 and 15 months of age, and most children have been infected by age 2. The hallmark pattern is a sudden high fever, often above 103°F (39.4°C), that lasts three to five days. The fever can be alarming for parents, but it usually resolves on its own.
Once the fever breaks, a rash often appears. The spots tend to be flat, pink, and neither itchy nor painful. They typically start on the chest, back, and belly, then spread to the neck and arms, sometimes reaching the legs and face. The rash lasts anywhere from a few hours to a couple of days before fading. Not every child develops the rash, and some infections cause only mild fever or no noticeable symptoms at all.
Reactivation in Transplant Patients
In healthy people, the immune system keeps dormant HHV-6 in check for life. The virus becomes a clinical concern primarily in people whose immune systems are deliberately suppressed, particularly recipients of bone marrow (hematopoietic cell) transplants and, to a lesser extent, solid organ transplants.
HHV-6B is the single most frequent infectious cause of encephalitis (brain inflammation) after bone marrow transplantation. This complication develops in approximately 1% of transplant recipients overall, but the risk rises significantly for those who receive cord blood transplants or transplants with reduced T-cell content. Symptoms can include confusion, memory problems, and seizures. HHV-6B reactivation has also been linked to a higher risk of graft-versus-host disease, a condition where transplanted immune cells attack the recipient’s body, as well as post-transplant pneumonia and episodes of fever and rash.
Treatment for serious HHV-6 reactivation relies on antiviral medications that block the virus from replicating. These are given intravenously in a hospital setting and are typically reserved for confirmed or strongly suspected encephalitis, since the drugs carry significant side effects. Guidelines published in 2025 by the American Society for Transplantation and Cellular Therapy provide updated recommendations for managing HHV-6B after transplant procedures.
Links to Neurological Diseases
Because HHV-6A in particular thrives in nerve tissue, researchers have been investigating whether the virus plays a role in chronic neurological conditions like multiple sclerosis and Alzheimer’s disease. The associations are real but not yet proven to be causal, and distinguishing active viral damage from the mere presence of dormant virus in the brain remains one of the biggest challenges in this field.
In Alzheimer’s research, several proposed mechanisms have emerged. HHV-6 antigens can activate the brain’s resident immune cells (microglia), triggering the release of inflammatory signals. These signals stimulate production of beta-amyloid peptides, the proteins that clump into the plaques characteristic of Alzheimer’s. The virus may also alter how the brain processes amyloid precursor protein and promote abnormal changes to tau, another protein involved in Alzheimer’s-related tangles. Whether the virus initiates these processes or simply accelerates damage that was already underway is an open question.
Multiple sclerosis has similarly been linked to HHV-6 in epidemiological studies, with the virus found more frequently in the brain lesions of MS patients. HHV-6A’s ability to infect the cells that produce myelin, the insulating sheath around nerve fibers whose destruction is the central feature of MS, provides a biologically plausible mechanism. Still, nearly everyone carries HHV-6, so proving that it triggers disease in a subset of people requires more than correlation.
Diagnosis and Testing Challenges
Roseola in children is usually diagnosed based on symptoms alone. No lab test is needed when a toddler follows the classic pattern of high fever followed by a rash.
In transplant patients or other immunocompromised individuals, diagnosis gets more complicated. PCR testing, which detects viral DNA in blood or spinal fluid, is the primary tool. But interpreting results is tricky. HHV-6 DNA can be found in the spinal fluid of people with no neurological symptoms at all, so a positive test does not automatically mean the virus is causing disease. And in the roughly 1% of people with chromosomally integrated HHV-6, every cell in the body contains viral DNA, producing consistently high PCR readings that can look identical to an active, dangerous infection. Current diagnostic techniques struggle to reliably distinguish active infection from latency or chromosomal integration, making clinical context essential for interpreting any test result.
Blood antibody tests (serology) are less useful in acute settings because most adults already have antibodies from childhood infection. A positive antibody test tells you almost nothing about whether the virus is currently active. Serology is mainly helpful in research and epidemiological studies rather than individual patient care.