A:   The basis of vaccination is to have controlled exposure of the animal to certain proteins (called antigens) of the disease causing agents. The animal becomes primed with an immune response and prepares infection fighting cells and circulating antibodies (proteins that stick to the antigens) Later, when exposed to the disease causing organism, the animal is prepared with an effective immune response against the agent. Typically vaccination is done with killed organisms or live organisms that are not capable of causing disease.

Sometimes autogenous vaccines are prepared from disease causing organisms originating from the same herd (as you describe). However, unlike what you describe, autogenous vaccines are prepared, or administered in such a way that the agent is unable to cause disease.

Injecting bacteria-contaminated milk into the tissue of a cow is not a good idea. I would expect the result would be a bacterial infection and abscess at the injection site. I am not aware of any clinical trials that support this the idea. Rarely is vaccination useful for treating an existing infection.