Vaccination
How does vaccination work?
Animals can be vaccinated against a number of dangerous infectious diseases. This is how vaccination works: inactivated or modified (to avoid causing actual disease) bacteria or viruses are administered to the animal by injection (with a sterile syringe and needle). The immune system then becomes activated and produces specific antibodies. These antibodies initiate a cascade which destroys the pathogens, such that in a later case of "true " infection, certain immune system cells called memory cells "remember" the infection. Now the immune system can react much faster, and can successfully conquer the invaders, often even before the first symptoms appear.
Sustained protection
Two or more doses are usually needed to initiate an adequate immune response. Over time, however, the amount of antibodies produced by the activated immune system (the antibody titer) gradually declines. Therefore, a booster shot is needed at regular intervals. Protection against some diseases such as tetanus and rabies can be accomplished by boostering once a year. Others require more frequent intervals to provide adequate protection.