Personal understanding: Vaccinations are like disaster drills, where the immune system builds up preventive protection through minor attack drills with inactivated or attenuated vaccines, so that when a real disaster strikes, the immune system can effectively target it without making a mess of it.
Of course, vaccination is not recommended for children who are infected, immunodeficient, or allergic, because live attenuated vaccines can cause adverse reactions in weak children even when the body is completely defenseless.
This article is published with the permission of Dr. K. Liang.