Possibility of mixed infection arising in encounter of agents from different families (virus-virus, virus-bacteria, virus-protozoa) was demonstrated on 103 cases of chronic tick-borne encephalitis (TBE) association with other acute and chronic infection. The addition of influenza, tuberculosis, pyogenic infection in most cases entails a relapse of TBE or promotes a primary-progressive course of the disease. TBE association with rheumatic fever relieves pain symptoms. It is emphasized that mixed infections used to bring suppression of immune reactivity thus necessitating serological and virological monitoring of the disease run.