History 50 year-old man with fever and headache starting one week after returning from his vacation in Thailand. His general practitioner prescribed amoxicillin/clavulanic acid, without further analyses to pinpoint the infection. Examinations The examination of cerebro-spinal fluid was crucial for the final diagnosis. Lumbar puncture demonstrated a predominantly mononuclear pleocytosis of 80 cells/µl (< 5) with an elevated protein of 782 mg/l (< 450); glucose and lactate were within normal limits. Treatment and course Initially we tried to treat a broad range of organisms potentially causing meningitis or encephalitis. Typical bacteria and viruses endemic to Switzerland were not found, thus anti-infective treatment was stopped. Also the search for malaria, HIV, Chikungunya and Dengue infections yielded negative results. After 10 days we received a positive serologic test for Japanese encephalitis virus (JEV). Conclusions The Japanese encephalitis virus is transmitted by mosquitoes and is epidemiologically one of the most important transmissible neurologic diseases in Asia. Although only a minority of infected patients are gravely ill, their sequelae and death toll are considerable. Since 2009 a well-tolerated vaccine is available.