Injecting an infectious organism into a person allows the body to make protective antibodies, protein substances that weaken germs.   The substance injected is a vaccine. It protects against most diseases caused by viruses.  Vaccines are usually used in the first years of life to prevent diphtheria, tetanus, poliomyelitis & whooping cough.
 
 

Vaccines contain dead organisms.  They are usually introduced in the skin by inoculation, serum injected.   Protection lasts for different amounts of time for different diseases.  The first people immunized are those most likely to get the disease.  Not every person needs to be immunized against all diseases.  Some diseases, like German measles, are treated by public health workers as mass immunizations. School age children as well as women of childbearing age are immunized in mass immunizations.

Immunization is the main weapon the U.S. has to fight against infectious disease.  Many states have a rule that children could not enter school without a certificate of measles immunization.  It has been proven that mass inoculation rules have reduced certain diseases in the U.S. 

New vaccines are still being developed, such as for rabies, hepatitis B & pneumonia - causing bacteria.  For Hepatitis a serum contains antibodies is injected into humans instead of dead organisms.

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