The success of vaccines over the past several decades has been so immense that infectious diseases which were commonplace just two generations ago are so rare today that many people know little or nothing about them.

But vaccination is an active rather than a passive type of protection. If as a society we become too complacent about immunizations, infections that were very nearly eradicated will return.

We are reminded of this medical reality by the current outbreak of mumps in Union County, where four cases of the viral illness have been confirmed, and five others are presumptive. Union County health officials haven’t determined the source, or sources, of the mumps outbreak.

For perspective as to how rare mumps became after 1971, when the MMR vaccination — measles, mumps and rubella — was licensed in the U.S., Oregon’s health officials stopped reporting mumps in 1981 because the malady was so uncommon.

But that positive trend didn’t last. In 2006 the Oregon Health Authority again began tracking mumps cases following several outbreaks.

The disease remained rare for several years, with 20 cases statewide between 2010-15.

But from Jan. 1, 2016, through Feb. 25 of this year, Oregon reported 47 cases of mumps (36 of those in either Marion County or Washington County), an increase in the prevalence of the disease that mirrors national trends.

According to the Centers for Disease Control and Prevention, the increase in the number of mumps cases is due to several factors, including that the MMR vaccine (like all vaccines) is not 100 percent effective.

But the agency also notes, referring to the national trend, that: “As usual, a disproportionate number of cases have occurred among the unvaccinated and partially vaccinated.”

The “as usual” tells the tale not just with mumps, but with other vaccine-preventable diseases that, though still quite rare, have become more common over the past dozen years or so as vaccination rates, in certain areas, have declined.

The concept is “herd immunity.” It means that so long as a certain percentage of the population is properly vaccinated — typically around 90 percent, although the percentage varies depending on the disease — then the chances of the disease spreading are so remote that people who aren’t vaccinated in effect benefit from the protection created by those who are.

Herd immunity is important with all vaccine-preventable diseases. In the case of mumps, herd immunity helps protect babies, who typically don’t receive the MMR vaccine until they’re between 12 and 15 months old.

The bottom line here is that when you get yourself and your children vaccinated, you might also be protecting people you’ll never meet.

From the Baker City Herald editorial board. The board consists of publisher Kari Borgen, editor Jayson Jacoby and reporter Chris Collins.