Sore mouth is caused by a poxvirus (specifically orf virus) and is found all over the world. The scabs of infected animals contain virus, can fall off, remain in the environment and serve as a source of infection to susceptible animals. A flock can become infected through contaminated bedding, feed or trucks, or by direct contact with infected animals (e.g. replacements brought onto the operation or at shows).

Animals may become infected with sore mouth more than once in their lifetime although infections are likely to occur years apart. Young animals will have the most visible disease because they have not likely been exposed to the virus before and because their immune systems are still developing.