If they are in dirty environment and get mud on the udder all the time and are exposed to lots of bacteria etc they sometimes develop mastitis.
They will get a bit agitated when you try to milk them and the milk will have what looks like clotted cream in it in lumps. The teats can also be a bit inflamed.
Usual treatment is by inserting a syringe of antibiotic into the teat and dosing it up. Clears it up in a few days, you can't drink the milk for a week or so though.
Clean sleeping areas are important to keep it under control.
Another thing to keep an eye out for, although it doesn't ccur often and you really need to get on it QUICK is Milk Fever.
This sometimes happens when a cow calves and for some reason ALL of the nutrients it gets from its food go straight into the milk and the cow can deteriorate in a few hours to the point where it is too weak to stand. If left it will die quickly.
If this happens you need to get the calf away from it immediately and get a vet to the mother quick or you will lose it.
I don't know what causes it, and I have only seen it twice. Once the cow lived, the other one wasn't caught in time.