The New Scientist looks at the evidence for damage to memory caused by viral infections across a lifetime: "A group of aggressive viruses, including those responsible for common colds, polio and diarrhoea, may harm a crucial memory-processing region in the brain known as the hippocampus, researchers say. The viruses, called picornaviruses, infect more than a billion people worldwide each year - people contract two or three such infections per year on average. ... This fact may explain the severe memory problems seen in some elderly people who do not have neurodegenerative illnesses ... repeated severe colds might possibly cause damage to this brain region that accumulates over a person's lifetime. [Researchers] plan to conduct brain scanning tests to look for signs of such damage in people with a history of acute infections." In addition to the buildup of damage over a lifetime, we can speculate as to the role of an increasingly ineffective aging immune system.