Passive immunity lasts only as long as the antibodies survive in body fluids.
Booster shots are needed periodically to build up the passive immunity again.
Booster shots are needed after antibodies won't survive in body fluids.
What are needed after antibodies won't survive in body fluids?
  * booster shots
  * Veins and arteries.
  * chicken pox
  * Energy.
  * gun shots
  * kinetic energy
  * diseased tissue
  * turn to a doctor