Alphavirus transmitted by mosquitoes (Aedes, Culex, and Culiseta) from birds to humans; prototype alphavirus for laboratory study. Clinical features of infection are macular rash (starting on torso and spreading to limbs), arthralgias, low-grade fever, tendonitis, headache, fatigue, and paresthesias. Virus causes Ockelbo disease (Sweden), Pogosta disease (Finland), and Karelian fever (Russia).