I and my group heartily agree with Edward L. McNeil1 about the need to search for infectious agents in idiopathic diseases, although there is often the problem of cause and effect: Detection of an infectious agent in the diseased tissue does not necessarily mean that it is involved in the pathogenesis of the disease. However, in Alzheimer disease (AD) an infectious agent has indeed been sought—and found, and implicated directly as a cause. Using polymerase chain reaction (and taking extreme care against contamination), we detected herpes simplex virus type 1 (HSV1) in a high proportion of post mortem brains of both AD patients and age-matched normal people1. We subsequently found that in combination with the type 4 allele of the gene for apolipoprotein E (apoE-e4), HSV1 confers a strong risk of the disease. We have since confirmed the presence of the virus in brain by immunological methods....

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