Final answer:
When getting a vaccine, the virus or bacterium that causes the disease is either dead or inactivated, or it is present in such small quantities that it cannot cause the disease.
Step-by-step explanation:
A vaccine contains dead or weakened forms of disease-causing viruses or bacteria. These viruses or bacteria are inactivated or given in such small quantities that they do not cause any disease in the person.
Instead, they stimulate the immune system to develop resistance or immunity to the specific virus or bacteria by producing and storing memory cells.
When the active virus or bacteria enters the body in the future, the immune system is able to recognize and eliminate it quickly, preventing the disease from developing.