By decreasing herd immunity, they're raising the chance that a given disease will become endemic, and that others, particularly the young or immunosuppressed, will catch said disease.

Vaccines aren't 100% reliable, so your sick neighbor has a (small) chance of infecting you.

Two obvious and immediate ways.

1) From the article: "That means far more students are coming in contact with peers who are unvaccinated—a serious risk, since vaccines are not 100 percent effective."

2) Increasing taxes to deal with the huge extra onslaught of people getting sick.

TFA: "Herd immunity occurs when a few unvaccinated children are protected by the fact that almost everyone around is vaccinated and therefore cannot infect them. It's important for those for whom vaccines have not worked, those who have immune problems, or those who cannot be vaccinated due to specific health risks"

Pretty simple. You take your newborn baby to a doctor's office. Another older child is there as well who has not been vaccinated and is sick. He gets your newborn baby sick.