Dementia affects 1 in 10 Americans over 65.Image: Flavio Coelho/Getty Images Alzheimers diseases impact is growing as our population ages, but much of the diseases pathophysiology remains unclear. A study published last year in the journal Nature revealed a significant new finding about Alzheimers diseaseit could spread between individuals under unusual circumstances. The finding is one of several that link Alzheimers to rare prion diseases, a connection that could change how we prevent and treat dementia.A dangerous donationCraniopharyngioma is a benign brain tumor, meaning it isnt cancerous. However, these small growths, which squat in the base of the brain, still have devastating side effects. They open shop next to the pituitary gland, a source of critical hormones. In adults, this can affect sexual performance or menstrual cycles.Craniopharyngioma, illustration. Credit: VERONIKA ZAKHAROVA/SCIENCE PHOTO LIBRARY VERONIKA ZAKHAROVA/SCIENCE PHOTOIn children, the tumor can stunt growth in crucial years of life. Biologists found a breakthrough treatment for these children in the 1950shuman growth hormone. Usually released by the pituitary gland, the scientists found that human growth hormone could be extracted from other people and donated to those with craniopharyngioma. Donors could even give their hormones after death. However, over the 25 years that children received these donations, some patients lives changed for the worse.The problem with the treatment was one missing step. Most of the donation protocols required that clinicians sieve donated brain extract using a technique called size-exclusion chromatography. This separation ensured that large protein aggregates were absent from the final donation. One approach omitted this step, with disastrous consequences. Two hundred children developed Creutzfeldt-Jakob disease (CJD)a rare prion disease that causes rapid brain damage and dementia. Since the donation protocols had not screened thePrP protein out of the donated samples, some of the patients were sickened.In response, the use of cadaver-derived growth hormone was immediately and permanently suspended. But prion diseases can take decades to emerge, so researchers have monitored affected children throughout the rest of their lives.That explains why it took until 2024 for scientists to realize that prion disease wasnt all that had slipped through the net.Alzheimers can be inducedThe 2024study led by University College London professor John Collinge reported on five growth hormone recipients who had developed Alzheimers disease pathology at incredibly young agesone at just 39 years old. Collinges group had previously reported that some of the growth hormone batches that four members of the group received had been contaminated with amyloid beta and tau, two proteins that are heavily implicated in the onset of Alzheimers disease. Lary Walker, a neurologist and neurodegenerative disease researcher at Emory University, has argued for decades that the way Alzheimers disease spreads in the brain mimics the march of prion proteins. Walker explains to Popular Science that colleagues had often received his experimental work with the same hesitancythat findings in animal models may not be replicable in humans. This new study shows that these findings are replicable, according toWalker. The paper indicates that actual Alzheimers disease can be induced in all of its manifestations in humans, Walker added.Richard Knight, a prion researcher at the University of Edinburgh, explains that prion diseases and other neurodegenerative conditions are linked by shared mechanisms. They all involve the deposition of an abnormally folded normal protein in the tissue, Knight tells Popular Science. The conditions also usually all begin in one specific brain area and spread.But prion diseases differ from other conditions in their aggressiveness. Knight, who worked in the field during the outbreaks of CJD that affected the UK in the 1990s, points out that the disease can progress at a terrifying rate. CJD could begin in the cerebellum, affecting coordination, but then affect the whole brain in weeks. Alzheimers disease might spread over a decade or more, Knight explains.Transmission and transfusionKnight says that the method of transmission that Collinge identified is extremely unusual, and theres no evidence that Alzheimers can spread through medical procedures used today. He also conceded that if Alzheimers could spread more easily, it would be much harder to notice.An exploratory 2023 study published in the journal JAMA Internal Medicine raised the troubling prospect that blood transfusions could possibly transmit the amyloid-linked disease cerebral amyloid angiopathy(CAA). Prion diseases like CJD may also be spread via blood transfusions. The study authors wrote that their results require replication to interpret correctly, but it would be hard to determine if either CAA or Alzheimers disease, can be transmitted this way.Prion disease is incredibly rare. Knight explains that when several blood donors and their recipients both went on to develop prion disease, it quickly became apparent that it was no coincidence. But when 40 percent of people over age 90 have dementia, it becomes much more challenging to work out which cases may have been transmitted via blood, a patients genetics, or environmental factors.Exploring how disease-causing proteins spread is essential work, which could help improve treatments by pointing researchers toward early biomarkers of disease. Understanding these proteins could also answer pressing questions about risk factors for these conditions. While Alzheimers isnt infectious like diseases from avirus or bacteria, establishing how this disease moves can help us stop it altogether in the future.