In 1937 Chumakov participated in a scientific expedition to Khabarovsk region of Soviet Far East that was led by Professor Lev A. Zilber. Together with his colleagues he discovered the etiology of a new transmissible neurological disease called tick-borne encephalitis (TBE) and isolated the virus that causes it. He was accidentally infected with the virus and developed encephalitis that led to a permanent loss of hearing and paralysis of the right arm.