When the person is vaccinated against measles the vaccine contains half dead measle virus. our WBC fight against the disease and wins. so the body becomes armed against measles. next time when measles attack the person is not infected.
Vaccine is nothing but dead pathogens or live attenuated when it is given to a measles patient they are considered as antigens by the the body produces antibodies against them which are kept when once again when the pathogen attacks the body antibodies fight as so the person doesnt catch pathogen
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