Not all of them go on to develop dementia.
Researchers believe that the results of this study could help pave way for better treatment options for the disease. Joel Dudley, one of the authors and an associate professor of genetics and genomic sciences at the Icahn School of Medicine at Mt. Sinai, said this was "gas to the flame" of what we already knew about the viral connection with Alzheimer's disease.
Millions of people around the world are affected by Alzheimer's disease, but the disease remains to be a mysterious one.
"In the Alzheimer brain what are the microbes that matter, what are the microbes that trigger the plaque?"
"I don't think we can answer whether herpes viruses are a primary cause of Alzheimer's disease". Neuroscientist Keith Fargo of the Alzheimer's Association, not involved in the research, tells Rosenbaum that a lot of genetic factors are also at play in the disease, and it will take more studies to untangle how immune reactions, genetics and other factors play into its progression.
Readhead says around 90 percent of children in the US and the United Kingdom are exposed to these viruses in the first few years of life.The findings don't prove the viruses cause Alzheimer's, nor do they suggest it's contagious. To evaluate the robustness of their findings, the team incorporated a further 800 RNA sequencing samples collected by the Mayo Clinic and Rush Alzheimer's Disease Center, observing a persistent increase of HHV-6A and HHV-7 abundance in samples from individuals with Alzheimer's, thus replicating their main findings in two additional, independent, geographically dispersed cohorts.
Another possibility is that the two theories are both at work.
"The new study gives the results of a large-scale analysis of data from the post-mortem brain samples of Alzheimer's patients and "control" patients with normal brain function".
The study is by no means conclusive evidence that the viruses cause Alzheimer's. The findings could change how scientists look for ways to treat or prevent Alzheimer's.
Readhead said that there is a need for more research to determine the extent to which the viruses are involved in causing the disease. HSV-1 for example - one that causes cold sores - has been studied in neurodegenerative diseases. Rather, the findings show viral DNA sequences and activation of biological networks - the interrelated systems of DNA, RNA, proteins and metabolites - may interact with molecular, genetic and clinical aspects of Alzheimer's.
To their surprise, they found many interactions, suggesting that these viruses can switch on or odd the genes related to Alzheimer's.
"All these Alzheimer's brains in these separate, major brain banks have previously unsuspected substantial populations of herpesvirus genomes and that deserves an explanation wherever it falls in the pathogenesis", Gandy said.