(CNN) -- West Nile Virus has killed 15 people in northern Greece and sickened 158 others, the Hellenic Centre for Disease Control and Prevention reported Friday.
Thirty-nine people were hospitalized Friday, nine of them in intensive care, the agency said. Another 116 patients have been treated and discharged from the hospital.
West Nile Virus is usually transmitted by infected mosquitoes or blood transfusions. Severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis.
About 80 percent of people infected with the virus show no symptoms, health officials say.
Authorities in central Macedonia, in northern Greece where most cases have been reported, said they would step up spraying programs in an attempt to ward off mosquitoes.
Authorities also said they are taking steps to prevent transmission by blood transfusions. Blood donations in regions at high risk for West Nile Virus have been canceled and people leaving the area are encouraged not to donate blood for up to 28 days.
Sporadic cases of the disease have been found throughout Europe since the 1960s.