Viruses have long been associated with neurodegenerative diseases. An example of this is the herpes virus and its potential role in Alzheimer’s dementia. A study published in 2022 also provided the strongest to date Evidence that Epstein-Barr virus may cause multiple sclerosis. However, many of these studies only looked at a single virus and a specific brain disease.
Kristin Levine and her colleagues at the National Institutes of Health in Bethesda analyzed approximately 450,000 US electronic health records and found one Connection found between different viruses and an increased risk of neurodegenerative diseases such as Alzheimer’s or Parkinson’s. However, the experts point out that the data only show a correlation and it is still unclear whether or how the infections cause the disease to break out. One of the strongest associations was between encephalitis, an inflammation of the brain caused by several types of viruses, and Alzheimer’s. Such an infection increased the likelihood of having the form of dementia later in life by a factor of 31. Most other associations were weaker: people who had had the flu (influenza virus) and then pneumonia were four times more likely to develop Alzheimer’s in their lifetime than those who had no such infection according to medical records.
Kjetil Bjornevik, an epidemiologist at Harvard TH Chan School of Public Health in Boston, points out that Levine’s approach to using medical records could be problematic. As a result, only infections that were severe enough to justify a doctor’s visit were taken into account. The inclusion of milder cases could weaken the connections. It is also unclear whether a viral infection has led to a neurodegenerative disease or whether this makes it more susceptible to infections. Another theory is that viruses speed up molecular changes in the body that were already underway, says Cornelia van Duijn of the University of Oxford.