Is Alzheimer’s Caused By Viruses?

The theory that suggests common viral or bacterial infections could potentially cause Alzheimer’s disease has left many patients on edge. While this so-called “Fringe Theory” linking viruses and other microbes to Alzheimer’s was first proposed more than 30 years ago, it has gained traction over the past several years.

While we do know the immune system plays a key role in the development of Alzheimer’s disease, there is not enough evidence to support a hypothesis that suggests Alzheimer’s is caused by viruses or other types of infections.

Below, we provide information on the theory that Alzheimer’s is caused by viruses, detailing the infectious hypothesis, types of infections claimed to cause Alzheimer’s disease, and known factors involved in Alzheimer’s development.

Is Alzheimer’s Caused By Viruses?: The Infectious Hypothesis

The infectious hypothesis suggests that some sort of pathogen (such as a virus, bacteria, or prion) is the underlying root cause of Alzheimer’s disease. The theory stems primarily from evidence that some pathogens, such as herpesviruses, are more commonly found in the brains of Alzheimer’s disease patients compared to those without Alzheimer’s.

What Types of Infections Are Claimed to Cause Alzheimer’s?

A few of the infections and viruses being researched due to their connection to Alzheimer’s disease include herpes, pneumonia, and spirochete bacteria.

Herpes

Herpes simplex virus 1 (HSV-1), also known as oral herpes, causes an infection of the mouth in the form of cold sores. This virus stays in the body for life but remains dormant most of the time. While HSV-1 typically infects the body, it has been found in the brains of both healthy individuals and those with Alzheimer’s disease. However, the HSV-1 virus seems to be more common in those with Alzheimer’s disease who have a risk gene for Alzheimer’s, APOE ε4.

Pneumonia

Chlamydia pneumoniae is a respiratory pathogen that causes pneumonia and bronchitis. While it typically infects the respiratory tract, the passage formed by the mouth, nose, throat, and lungs, it can sometimes remain as a chronic infection inside cells. Because this bacteria has been found in the brains of those with Alzheimer’s disease, some researchers have hypothesized that the inflammation caused by the presence of this bacteria may contribute to underlying pathological mechanisms of Alzheimer’s.

Spirochete Bacteria

Spirochetes, such as Borrelia burgdorferi, which causes Lyme disease, and Porphyromonas gingivalis, which causes gum disease, have also been linked to Alzheimer’s disease. Spirochetes are significantly more common in the brains of those with Alzheimer’s disease compared to those without Alzheimer’s. Additionally, chronic gum disease has been identified as a risk factor for Alzheimer’s.

Alzheimer’s as a Multifactorial Disease

While there is currently no known causative agent for Alzheimer’s disease, there are many well-known risk factors. Alzheimer’s disease is believed to be multifactorial, meaning it is not caused by a single factor, like a virus, but rather develops from a combination of several factors, such as age, genetics, and lifestyle, as detailed below.

• Age: Increasing age is the greatest known risk factor for Alzheimer’s disease, though age itself does not cause Alzheimer’s disease.
• Genetic factors: Researchers have identified several genes that increase the risk of developing Alzheimer’s disease (risk genes) as well as genetic mutations within specific genes that guarantee the development of Alzheimer’s disease (deterministic genes), though the latter accounts for only 1% of Alzheimer’s cases.
• Lifestyle factors: Potentially modifiable risk factors for Alzheimer’s disease include, but are not limited to, poor diet and nutrition, a lack of regular exercise, heavy alcohol consumption, poor sleep, a lack of social and mental engagement, hearing loss, cataracts and other causes of visual impairment, vascular conditions, and depression.

Key Takeaways

The truth is you can find correlations between any two things. Just because viruses and other infections are more commonly found in patients with Alzheimer’s disease does not necessarily mean that Alzheimer’s is caused by viruses. Correlation does not imply causation. This is not to say that these viruses have no bearing on Alzheimer’s disease; however, it is more likely that the higher prevalence of such infections in Alzheimer’s disease patients is caused by the higher susceptibility to infection for those with the disease.

To better understand and address the breadth of factors involved in Alzheimer’s disease development, more research will be required in addition to the application of precision medicine approaches. Taking a more precise, personalized approach to neurological diseases, like Alzheimer’s disease, will give rise to a wide range of targeted drugs and therapies that can be effectively used in combination to treat specific neurocognitive impairments (impairments to cognitive and functional aspects of brain function) that present uniquely in neurological disease patients.