vaccine is prevention of disease against pathogens . the work of vaccine is to boost up the body immunity towards pathogens. e.g. anti-rabies vaccine , MMR , Oral polio vaccine etc. 
   a child was unknown from when the pathogen attack on his body but .
                           in the process of vaccine the immune cells is trained for future infections . for e.g. when a child got IPV vaccine by birth then he/she  is ready to fight because   the vaccine charge his immune system for the future  infection and when immune system first sees an infectious microbe , it responds against  it an then remembers it specifically . so the next time that pacxticular microbe , or its close relatives enter the body then the immune system responds with greater force and velocity and this terminates the infection more quickly than the FIRST TIME . immune system also develops our memory by putting that mimics to the microbe  that prevents us from subsequent exposure to infecting microbe . 
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Vaccine is nothin but the causative organism like of tge bacteria or virus is taken n injected into the body in a weak state..... our body releases antibodies n serum to fight it.... since tge orfanism injected was weak it gets killed n removed..... While this procesz gets stored in memory cells n defends against real organisms attacj
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