Campylobacter foetus venerealis
Campylobacter foetus venerealis is a sexually transmitted bacteria that colonises the anterior vagina of infected cows and the preputial folds of infected bulls.
Infection with C. foetus venerealis can cause infertility and abortions. It is passed from cow to bull to cow at serving, so it's more important in herds using natural service with a bull. Infected cows, in the absence of a bull, will eventually develop an immunity to the organism and eliminate the infection but infected bulls will only eliminate the infection following treatment which is not easy nor always successful.
It is sensible to try and and use virgin bulls - Discuss with your vet if trying a non virgin bull.