Here’s why.
vaccination is not 100% effective.
They may be immunocompromised and at risk for measles.
They may have an infant too young to be vaccinated.
Get it?
When my older brother was 12 years old, he came down with the mumps and seemed to be getting over it but then developed encephalitis and meningitis that very nearly killed him. He lapsed into a coma, the doctors were not optimistic that he would live and if he did, they feared permanent brain damage. My brother did eventually fully recover and didnt have any permanent brain damage but he was hospitalized for nearly 2 months and it took quite a while for him to relearn how to walk and talk and feed himself, his motor skills were so screwed up. I have family photos of my brother before and after and it doesnt look like the same person. The before photos show a rather robust, athletic and healthy kid, the shortly after photos shows a very skinny and pale kid. I was just a baby but you can be sure that after what happened to my brother, that my parents made sure that I got all my vaccinations, at least all the ones that were available at the time.
What many people dont understand about vaccinations is the herd immunity effect. You not only get vaccinated to protect yourself, you also get it to protect others; people who might have compromised immune systems such as cancer patients or those who cant get vaccinated because of allergies or other health problems, those too young to get vaccinated, those who had vaccinations as a child but never got the booster shots that are necessary for some diseases or those for whom for whatever reason, the vaccination didnt take or they only recently got the vaccination but hadnt yet developed the optimal immune response. While most vaccinations take and are highly effective, no vaccine is 100% effective 100% of the time for 100% of the people who received them.
I was one of those who got vaccinated for measles about a year before I got it but I still came down with it, the vaccination didnt take. But if the person, my school classmate who infected me had been vaccinated, he wouldnt have contracted it and he wouldnt have passed it on to me. Fortunately while I remember being very sick, the blinding headache, not being able to tolerate light, being very weak and missing a week of school was what I remember most, I recovered without any complications. One of my classmates wasnt vaccinated was not so fortunate. She didnt die but developed pneumonia and spent a month in the hospital.
Now days we have people here in the US who are refusing to get their children vaccinated over false fears of vaccinations causing Autism or incorrectly believing that all vaccinations are made from aborted babies (not true), and then we have those who eschew modern medicine all together opting for natural cures, herbal remedies or the misguided beliefs that childhood diseases are some sort of normal rite of passage and basically harmless.