Cancer and Alzheimer's disease are two of the most feared diagnoses in medicine, but they rarely strike the same person. For years, epidemiologists have noticed that people with cancer seem less likely to develop Alzheimer's, and those with Alzheimer's are less likely to get cancer, but nobody could explain why. In the tumour‑bearing mice, cystatin‑C activated a sensor on microglia known as Trem2, effectively switching them into a more aggressive, plaque‑clearing state. Surprising trade-offsAt first glance, the idea that a cancer could "help" protect the brain from dementia sounds almost perverse. Mouse models of Alzheimer's capture some features of the disease, particularly amyloid plaques, but they do not fully reproduce the complexity of human dementia.