AURORA, Colo . (Oct. 19, 2026) – A very common virus may do more than cause cold sores.
Researchers at the University of Colorado Anschutz have discovered how herpes simplex virus type 1 (HSV-1), the virus that causes cold sores, can fundamentally change how the brain’s immune cells respond to infection, revealing a potentially important connection between viral infection, chronic inflammation and Alzheimer’s Disease.
The findings, published today in PLOS Pathogens , reveal that HSV-1 activates powerful antiviral defenses in the brain’s specialized immune cells, called microglia. But this response may come at a cost. As these cells shift towards fighting infection, they lose the ability to maintain a healthy brain.
“Microglia act as both the brain's security system and its maintenance crew,” said senior author Christy Niemeyer, PhD, assistant professor of neurology at CU Anschutz. “When HSV-1 enters the brain, these cells shift into emergency defense mode. While this helps fight the virus, it may also compromise their ability to perform the everyday functions, even after the virus is gone.”
HSV-1 infects more than 60% of people and remains in the body for life. Although serious complications are rare, growing evidence links HSV-1 to the chronic inflammation that may trigger Alzheimer’s disease. However, it is not clear how the virus may trigger and maintain this inflammation.
A virus that changes the brain's immune response
Using advanced molecular techniques, researchers examined how individual brain cells responded to HSV-1 infection and where these changes occurred.
They discovered that infection produced distinct populations of microglia dominated by interferon signaling, a powerful antiviral defense mechanism. At the same time, genes involved in the cells' normal protective and maintenance functions became less active.
Importantly, these changes were concentrated in brain regions where the virus was detected, suggesting that HSV-1 creates localized areas of heightened immune activity.
The researchers also found changes in chromatin , the material that packages DNA inside cells. These changes can affect how easily genes are turned on or off.
"What makes these findings particularly interesting is that the virus isn't simply triggering inflammation," Niemeyer said. "It's changing the molecular programs that control how these immune cells function. An important question is whether these changes persist after infection."
The findings provide a potential explanation for how viral infections could contribute to Alzheimer’s disease and other neurodegenerative disorders.
“Interestingly, the way that HSV-1 finely tunes the immune response in microglia, resemble patterns found in response to other factors that drive Alzheimer's disease,” said co-senior author Kimberley Bruce, PhD, associate professor of medicine at CU Anschutz. “Although we have not established that HSV-1 causes Alzheimer's, it highlights the immune programming of microglia as a common factor linking infection, aging and genetic risk factors.”
Niemeyer said Alzheimer's disease is influenced by multiple genetic and environmental factors.
"Understanding how a common virus changes the brain's immune environment may help us identify why some individuals become more vulnerable to neurodegeneration," she said.
The researchers next plan to investigate whether these immune changes persist after infection and how repeated viral reactivation may influence long-term brain health.
The other co-authors include: Seth Frietze, Cameron Lunn, Dean Oldham, Joseph R. Boyd, Andrew N. Bubak, Amalia Bustillos Saucedo, Maria A. Nagel, Diego Restrepo, Kimberley D. Bruce, Christy S. Niemeyer
The University of Colorado Anschutz is a world-class academic medical campus leading transformative advances in science, medicine, education and patient care. The campus includes the University of Colorado’s health professional schools, more than 60 centers and institutes, and two nationally ranked independent hospitals – UCHealth University of Colorado Hospital and Children’s Hospital Colorado – which see nearly three million adult and pediatric patient visits each year. Innovative, interconnected and highly collaborative, CU Anschutz delivers life-changing treatments, exceptional patient care and top-tier professional training. The campus conducts world-renowned research supported by $890 million in funding, including $762 million in sponsored awards and $128 million in philanthropic gifts for research.
Find the latest CU Anschutz news here: http://www.cuanschutztoday.org/
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PLOS Pathogens
Animals
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