A few recent papers have, collectively, added evidence for persistent viral infection to be a significant contributing cause of Alzheimer’s disease. A number of viruses in the herpesvirus family are prevalent in the population but cause few obvious symptoms, such as HSV-1 and cytomegalovirus (CMV). Some of these, particularly CMV, are already under suspicion as being the cause of long-term dysfunction in the immune system. Viral infection is an attractive way to explain why only some of the people who exhibit all of the known risk factors for Alzheimer’s disease actually go on to develop the full clinical manifestation of the condition. The proportion of the population with latent infection is high, but not too high: other candidate differentiating factors are a lot less convincing because either the population size is too small, or near everyone has it.
How can viral infections contribute to the development of Alzheimer’s disease? The amyloid cascade hypothesis tells us that, in the first early stages of Alzheimer’s disease, amyloid-β accumulates in the brain, producing only comparatively minor symptoms of degeneration. In later life, this accumulation reaches a critical point that causes tau protein to alter and form solid neurofibrillary tangles in significant amounts.