I shouldn't see any cases of pertussis ("whooping cough"), but I do. We have a safe, effective and affordable vaccine. But still, people are getting this disease. In the age group I see (adults), immunity has often waned, and if they haven't been revaccinated, they can get the disease and pass it on. In adults it often looks like a cold, but not in kids. Most properly vaccinated children are immune and remain so until there little airways are large enough to cope with the illness. But a certain percentage of kids either don't get vaccinated or aren't successfully vaccinated, leaving them vulnerable to a disease that shouldn't even exist at a measurable rate.
In small children, with their little airways, pertussis often leads to hospitalization, and not infrequently kills. It kills children who should never have been put at risk, either because their parents refused to vaccinate them, or people around them did and gave them the disease.
Children and adults who should be vaccinated but are not represent a public health failure, and in the case of parents who deny their kids the vaccine, an ethical failure as well.
As a reminder, this is what pertussis looks like in little kids.