About 4.5 million Americans have Alzheimer's disease, and, with the aging population, many more are likely to contract it in the future. Despite an abundance of studies, the causes of Alzheimer's are not understood. One of the more controversial theories is that bacteria or viruses can cause the disease. A group of scientists from the University of Manchester in the United Kingdom and the Philadelphia College of Osteopathic Medicine laid out the case for a pathogenic cause.

What the researchers wanted to know: What is the evidence that supports viral or bacterial infection as a cause of Alzheimer's disease?

What they did: The scientists summarized data from numerous published studies to boost their argument that the herpes simplex type 1 virus and the Chlamydia pneumoniae bacterium (which is from the same genus as the Chlamydia that causes an STD but is not the same bacterium) could contribute to the onset of Alzheimer's disease.

What they found: A significant body of evidence on Alzheimer's studies and herpes or pneumonia infection suggests that these pathogens could play a role in the disease. Doctors have found evidence of both infections in the brain tissues of people who have died due to Alzheimer's. In one study, 90 percent of the Alzheimer's brains contained the pneumonia bacteria, while only 5 percent of non-Alzheimer's brains contained the same bacteria. In another study, scientists found that a gene strongly associated with Alzheimer's also put people at greater risk for cold sores—one symptom of a herpes infection. The authors suspect that both of these pathogens live in the body for some time and make their way up to the brain, where they cause inflammation, which in turn causes the build up of proteins associated with cognitive decline. In mice, infection with the pneumonia virus caused a build-up of the protein plaques thought to cause the memory loss in Alzheimer's disease.

What it means to you: This analysis could point to new areas for researchers to search for a cure, or at least treatment, for Alzheimer's. There is a small amount of evidence that suggests that vaccination against various viruses protects against Alzheimer's, but more studies are needed to make this result conclusive.

Caveats: The authors acknowledge that there are many factors that contribute to Alzheimer's disease, and many Alzheimer's researchers don't think that pathogens are a cause. However, some of the dissenters reviewed this paper before it was published and passed it through.

Find out more: There is a good article about the role of infection and inflammation on the Alzheimer's Information Site.

A general but comprehensive fact sheet about the disease is on the National Institutes of Health website.

Read the article: Itzhaki, R.F., Wozniak, M.A., Appelt, D.M., Balin, B.J. "Infiltration of ohe Brain by Pathogens Causes Alzheimer's Disease." Neurobiology of Aging. May/June 2004, Vol. 25, No. 5, pp. 619–627.

Abstract online: www.ncbi.nlm.nih.gov