A 27-year-old woman presented with rapid cognitive decline over 6 months. She did not have any significant past medical history. She presented with ideomotor apraxia, poor mathematical calculation, and severe short-term memory loss. Deep tendon reflexes were symmetrically brisk through all extremities and episodes of dystonic contraction of the left hand were observed. Brain MRI revealed global atrophy, especially of the hippocampi, without contrast enhancement. CSF analysis revealed 7 white blood cells (WBC) per mm³ (50% neutrophils and 50% lymphocytes), protein level of 73 mg/dL, glucose level of 67 mg/dL (peripheral glucose 92 mg/dL), negative VDRL, and negative cultures for bacteria, fungi and mycobacteria. RT-PCR for Herpes simplex virus 1-2 were negative in the CSF. Continuous video-EEG was unremarkable. The patient´s cognition continued to rapidly decline over the following weeks, and evolved into an akinetic mute state. She was treated with methylprednisolone and intravenous immunoglobulin, without improvement. CSF studies were repeated, showing an increase of CSF protein (93 mg/dL) and a decline in WBC (1 cell/mm³). Repeat MRI scans revealed progressive cerebral atrophy and subtle increased FLAIR signal in the bilateral mesial temporal lobes, without contrast enhancement. The patient was extensively tested for several causes of rapidly progressive dementia, but all tests were unrevealing. Serological testing was performed for Dengue virus, Chikungunya and Zika virus. ZIKV ELISA IgG/IgM antibody were positive in the blood and CSF, but negative for other arboviruses. PCR-RT for Zika virus was negative in the CSF and blood. The patient´s family declined performing brain biopsy.