Mastitis in one or more mammary glands is caused by a variety of bacteria or it may be secondary to other diseases. It is a common condition that occurs sporadically in individual sows or sometimes as herd outbreaks. It starts around farrowing and becomes clinically evident up to 12 hours later. It can arise because bacteria have gained entry to one or more mammary glands for the first time, or it may be a flare-up of a long-standing sub-clinical latent infection. The route of entry of the bacteria is probably the teat orifice but it may be from the blood stream or by injection on piglets' teeth. It also commonly occurs at weaning time.