Ticks, specifically hard ticks of the family Ixodidae, transmit tick-borne encephalitis (TBE) virus. Ticks become infected when they feed on blood from infected vertebrate hosts, particularly small rodents, and can subsequently transmit the virus during the next blood meal. Ticks can also get the virus transtadially (from larva to nymph to adult ticks), transovarially (from adult female tick to eggs), or when cofeeding on infected animals. The highest period of tick activity and risk of spread is during the warmer months from April through November.
Infection also may follow consumption of unpasteurized milk or cheese from infected goats, sheep, or cows. Other rare modes of transmission include through slaughtering of infected animals, blood transfusion, organ transplantation, breastfeeding, or from exposure to the virus in the laboratory.