It has also been found that the TLR-4 levels are enhanced in COVID-19 patients, in support of the relationship between COVID-19 and PD.118 The neurodegeneration due to SARS-CoV-2 could be triggered by the binding of SARS-CoV-2 spike protein to TLR4, affecting the dopaminergic neurons in the nigrostriatal system. Here, TLR4 is linked to Parkinson disease.