The majority of infections with BVD virus occur after birth. In this case animals become acutely or transiently infected before recovering and becoming virus-negative, typically within 3 weeks or less. Transient infections may occur without obvious clinical signs but depending on age, sex and immunity they can lead to a range of reproductive problems, including abortion and poor calf health with scours and pneumonias that respond poorly to treatment. Infection of the unborn calf between approximately 30 and 120 days of pregnancy will result in it becoming persistently infected (PI) for the rest of its life with BVD virus if the calf is not aborted. If a calf is not PI at birth it will never be PI.