During my studies for the pediatric board exams, I learned a lot about diseases I never expected to encounter. Some were incredibly rare. Others were isolated to corners of the globe where I never intended to practice. And some had been so well-controlled by vaccines that it no longer seemed like I would ever actually see a case during my career.

Perhaps it is naive, but I still hope that expectation holds true. I hope I never actually see a case of tetanus. I hope epiglottitis remains something I know how to recognize but never come across. And I certainly hope the geopolitical catastrophe of a single case of smallpox is something nobody ever lives to witness.

Until recently, measles would have been on my list of “theoretical but not actual” clinical considerations. Given the success of the MMR vaccine in eliminating the disease from the United States, I assumed I could file “Koplik spots” in the back of my brain alongside the signs of scurvy.

And then came the measles outbreaks. Last year there were eleven outbreaks, and 2014 isn’t shaping up any better. Given that—thanks to low vaccine rates—measles is now endemic in the United Kingdom, another outbreak is only a plane ride away when the destination is a vulnerable community.

One of the more infuriating complaints vaccine-refusers make about immunizations is that they’re not 100% effective, so why administer them to your children? Leaving aside the grossly unrealistic belief that any medical intervention is 100% effective for all recipients, this question indicates a deep misunderstanding of “herd immunity.” (It’s ironic that vaccine-deniers understand herd immunity so poorly, as it’s what they rely on to keep their own children free of disease.) Since no vaccine creates perfect immunity in everyone who receives it (which any competent medical provider will readily admit), it’s necessary for a certain percentage of the population to be vaccinated to protect the population as a whole. If enough people are protected, when a contagion enters the community it won’t spread even if not everyone who received the vaccine responded appropriately.

With measles already back, what else could we expect if vaccination rates fall below levels necessary to prevent outbreaks?

Mumps

Like measles, mumps is another vaccine-preventable illness that’s already back in the news. In fact, there’s an outbreak at The Ohio State University right now.

Mumps is a viral infection. The hallmark of the disease is painful swelling of the parotid glands (which are the salivary glands on either side of the cheeks), often accompanied by fever and malaise. Swelling can involve one or both glands. Other symptoms can include painful inflammation of the testicles, breasts, and ovaries. Rarely, severe infections can lead to inflammation of the brain or meningitis.

As with almost all vaccine-preventable illnesses, mumps used to be very common, but had a precipitous drop in infections with licensure of the first vaccine. It is currently administered as one of the components of the MMR vaccine (along with measles and rubella). Like some pertussis outbreaks, the clusters of new mumps cases that occur every few years (usually on college campuses) can’t be pinned entirely on vaccine-refusers. Reports from the Ohio outbreak indicate that most of those affected had received at least one mumps vaccine (though complete protection requires two shots), which shows that vaccines aren’t always 100% effective.

However, if we don’t want to go from “occasional, contained outbreak” to “common childhood illness” all over again, it’s important that all children be fully vaccinated to confer as much protection to public health as possible.

Pertussis

We may as well begin with another preventable illness that’s already coming back. Pertussis, or “whooping cough,” is caused by the bacterium Bordetella pertussis. It starts with symptoms that look a lot like a common cold, which then turn into weeks of uncontrollable coughing. The cough can be so bad it cracks ribs.

Awful as those symptoms may be, other outcomes of infection can be even worse. In infants under 12 months, the only symptom of the disease can be apnea—babies infected with pertussis can simply stop breathing as the first and only sign they’ve been infected.

With the advent of the first pertussis vaccine, rates of the disease dropped dramatically. However, unlike measles, it has never been considered eradicated within the United States. Over the past few years, the usual cyclic trend of new cases has been spiking higher and higher.

In the case of pertussis, the blame for increasing incidence of new infections may not be entirely the fault of vaccine-deniers. During the 1990s, the vaccine against whooping cough was changed to lower the risk of side effects like fever, and the newer vaccine may not confer immunity that is as broad or lasting. With that said, there is clear evidence that communities with a high rate of vaccine refusal are more likely to experience an outbreak.

Of course, why a person would choose to avoid a safe vaccine against a disease that kills infants without warning remains an open question.

Diphtheria

Parents who refuse pertussis vaccination aren’t keeping their children susceptible to that disease alone. The vaccine that protects against it (known as the DTaP) is a combination vaccine with components that protect against two other diseases, tetanus and diphtheria. Tetanus is a terrible illness, but one that is not spread from person to person. Diphtheria, on the other hand, is perfectly capable of causing an outbreak in a vulnerable population.

The “strangling angel” of children, diphtheria is a bacterial illness that causes a thick build-up of material in the upper airway, which can make it increasingly difficult to breathe. The same toxin that causes this “pseudomembrane” to form can also damage organs like the heart and kidneys. Death, which used to occur in roughly half the cases of the disease, is caused by any one of these factors.

Thankfully, diphtheria has been essentially eliminated in the United States. However, DTaP vaccination rates in this country are below federal goals. The disease still strikes thousands of people worldwide every year, including many countries in Africa and Asia. Travel from an endemic area to an under-vaccinated population in the United States is a distinct possibility.

Invasive Haemophilus influenzae

Haemophilus influenzae disease isn’t something you hear much about. Shortened in medical parlance to H. flu, it bears no relation to the viral infection influenza that strikes every winter. Rather, it is a collection of different illnesses caused by the same bacterium, varying by which organ system is most affected.

Infections can strike joints, airways, the lungs, the brain and the tissues lining the spinal cord, or the bloodstream. Up to 6% of cases are fatal, and 20% of patients who develop meningitis as a result of infection can suffer permanent neurological damage or deafness.

Thankfully, vaccination against the most virulent strain of H. flu (type B) has been available since the mid-1980s, and cases of invasive H. flu have become very rare since the “HiB” shot became part of the routine schedule. Unfortunately, immunizations rates for this disease also fall short of federal guidelines.

Like pertussis, H. flu has never been fully eliminated within the United States. There was a small outbreak in Pennsylvania several years ago that sickened seven children, three of whom died. There was a different outbreak in Minnesota the year afterward. Both of those outbreaks involved un- or under-vaccinated children.

Outbreaks of other vaccine-preventable illnesses seem less likely. Polio, for example, has been eradicated in the Western hemisphere thanks to a vigorous international immunization program. But even a well-controlled infection can recur if the population becomes vulnerable again, as occurred in Syria after the civil war there caused vaccination rates to plummet. Old interventions won’t keep protecting us if they aren’t maintained.

Vaccines are the best protection we have against the spread of devastating and once-common illnesses. Administered appropriately, they are a safe and effective means of keeping our children and ourselves healthy. But if people stop using them, those illnesses will leave the history textbooks where they belong and show up more and more on our front pages.