Final answer:
A vaccine is a substance made from particles of microorganisms that stimulate the immune system and develop immunity to specific viruses or bacteria.
Step-by-step explanation:
A vaccine is a substance made from particles of microorganisms that are either weakened or inactivated. It is used to stimulate the immune system and develop immunity to specific viruses or bacteria. Vaccines can be made using traditional methods with weakened or inactive forms of microorganisms, or through modern techniques that involve cloning genes into vectors to produce specific substances.