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Over 150 years ago, scientists proved that invisible germs could cause contagious diseases such as cholera, typhoid and tuberculosis. The role of microbes in these diseases quickly gained wide acceptance, but the “germ theory” has continued to surprise ever since – with huge implications for many seemingly unrelated areas of medicine.
It wasn’t until the 1980s, after all, that two Australian scientists discovered that Helicobacter pylori triggers stomach ulcers. Before that, doctors attributed the illness to stress, cigarettes and alcohol. Contemporary scientists viewed the idea as “absurd,” but it ultimately won the Nobel Prize in Physiology or Medicine in 2005.
Equally controversial has been the discovery that the human papillomavirus can cause cervical cancer, but vaccines against the infection are now saving thousands of lives. Scientists now estimate that about 12% of all human cancers are caused by viruses.
We may be witnessing a similar revolution in our understanding of Alzheimer’s disease. Lifestyle and genetic factors certainly play a role in the development of the disease. But it increasingly seems possible that some common viruses and bacteria – the ones that give us cold sores and gum disease – can, in the long run, trigger neural tissue death and constant cognitive decline. If so, infections can be one of the main causes of dementia.
Like germline theories of ulcers and cancers, this hypothesis was once considered some kind of heresy – yet a series of compelling findings have sparked renewed interest in microbes’ contributions to dementia. “There is a tremendous amount of work going on now, compared to five years ago,” says Ruth Itzhaki, professor emeritus at the University of Manchester in the UK, who has spent three decades studying the role of infection in Alzheimer’s disease.
The hypothesis inspired a clinical trial of a drug that could target the infection before it decimates the brain, drastically reducing the risk of senility in the elderly. And there could soon be many other exciting new treatments in the pipeline.
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A new understanding of Alzheimer’s disease couldn’t come soon enough. Although there are many forms of dementia, Alzheimer’s disease causes about 60-70% of cases. Globally, this represents around seven million newly diagnosed people each year, who are in desperate need of new treatments to slow their decline.
The disease takes its name from the German doctor Alois Alzheimer. In 1906, he noticed the accumulation of plaques in the brain of a 55-year-old woman, Auguste Deter, who suffered from memory problems, speech disorders and unpredictable behavior. We now know that these plaques are made from a protein called beta-amyloid, and are thought to be toxic to brain cells and alter synaptic connections that are important for neural signaling. The build-up of beta-amyloid plaques can also cause tangles of another protein, tau, to form in cells, which in itself can lead to neuronal death, and it appears to be accompanied by widespread inflammation. in the brain, making the damage worse.
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