Answers

2016-04-06T21:54:25+05:30
Expose the immune system to a weakened or harmless version of the virus / organism immunity is required for, so it can start to produce antibodies to fight that virus before the harmful "real" version is encountered.
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2016-04-06T21:55:11+05:30
The release of antibodies after the injection of weak bacteria into the body to fool it.
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