Answer:
Vaccines train our immune systems to create proteins that fight disease, known as ‘antibodies’, just as would happen when we are exposed to a disease but – crucially – vaccines work without making us sick. Live, attenuated vaccines fight viruses and bacteria. These vaccines
contain a version of the living virus or bacteria that has been weakened
so that it does not cause serious disease in people with healthy immune
systems. Because live, attenuated vaccines are the closest thing to a
natural infection, they are good teachers for the immune system.
Examples of live, attenuated vaccines include measles, mumps, and
rubella vaccine (MMR) and varicella (chickenpox) vaccine. Even
though they are very effective, not everyone can receive these vaccines.
Children with weakened immune systems—for example, those who are
undergoing chemotherapy—cannot get live vaccines. Those are only some vaccines, of course not all vaccines inject a live virus into your body. And of course, there are so many vaccines because (unfortunately) there are so many diseases!