Microorganisms associated with increased risk of Parkinson's disease.

Parkinson's disease (PD) is a neurodegenerative disorder that affects more than 7 million people worldwide. Its aetiology is unknown, although the hypothesis of a genetic susceptibility to environmental agents is accepted. These environmental agents include fungi, bacteria, and viruses. Three microorganisms are directly associated with a significantly increased risk of developing Parkinson's disease: the fungal genus Malassezia, the bacterium Helicobacter pylori, and the hepatitis C virus. If the host is vulnerable due to genetic susceptibility or immune weakness, these microorganisms can access and infect the nervous system, causing chronic neuroinflammation with neurodegeneration. Other microorganisms show an epidemiological association with the disease, including the influenza type A, Japanese encephalitis type B, St Louis, and West Nile viruses. These viruses can affect the nervous system, causing encephalitis, which can result in parkinsonism. This article reviews the role of all these microorganisms in Parkinson's disease.