Why Some Diseases Are Making a Comeback

In the past century or so, humans have fought—and won—their fair share of battles with disease. Vaccines defeated smallpox. Antibiotics conquered scarlet fever. And insecticide scaled back mosquito-borne illnesses.

Despite these successes, some diseases appear to be making a comeback. Outbreaks of measles and mumps have made more than a few headlines of late, and once-lost pathogens like cholera are creeping back into medical histories. While the reasons behind the rise and fall of diseases are often complex and difficult to pin down, here are a few key reasons behind some of these resurgences.

Vaccine Refusal

Doctor examining young patient
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One of the greatest public health achievements in history, vaccines are credited for the massive decline of potentially dangerous diseases like measles and polio. Although the majority of families embrace vaccination, a growing number appear to be delaying or forgoing vaccines altogether due to a misunderstanding of the safety, effectiveness, and necessity of vaccination.

In Texas, for example, the number of students with nonmedical exemptions to school vaccine requirements has skyrocketed from just 10,404 in 2007 to 52,756 in 2017. While the overall vaccination rate for measles in Texas held fairly steady between 2007 and 2017 with a rate of about 97 percent for students, research shows that unvaccinated individuals tend to cluster in the same communities and schools, resulting in a breakdown of protective herd immunity and leaving those individuals vulnerable to outbreaks of disease.

In the case of Texas, more than 360 of the state's 1,745 independent school districts—or 21 percent—had measles vaccination rates below the 94 percent threshold recommended to achieve herd immunity, and at least five districts reported measles vaccination rates of 50 percent or less. If someone infected with measles were to enter those communities, the disease could spread like wild fire.

Measles is one of the most contagious diseases known to mankind. It was officially declared eliminated in the United States in 2000, but since then, dozens of outbreaks and thousands of cases have been reported—including an outbreak involving Disneyland that led to more than 300 cases in the United States and Canada.

According to a review published in JAMA, high numbers of vaccine refusers in a given community increases the risk of measles not just for unvaccinated individuals, but for vaccinated people, too. That's because no vaccine is 100 percent effective. Some people who receive the vaccine might not respond to it and could get sick anyway if they are exposed to the virus.

Unless the United States is able to increase vaccination rates in communities throughout the country, these outbreaks will likely continue.

Waning or Insufficient Immunity

Measles isn't the only vaccine-preventable disease seeing a resurgence. Cases of pertussis and mumps have also been on the rise, and while vaccine refusal is certainly a factor, there's another culprit potentially at play: insufficient or waning immunity.

Many of the individuals involved in recent outbreaks of mumps and pertussis have been at least partially vaccinated. Does that mean that the vaccine doesn't work? Not exactly.

Pertussis and mumps vaccines are about 80 percent effective when they're first given. As time goes on, however, research suggests that that immunity wanes, and more doses might be needed to protect against outbreaks.

Vaccines work by training your body to fight a particular pathogen, like a virus, bacteria, or toxin. The immune system creates antibodies to fight off the vaccine, and then stores away the info in case they come in contact with the disease in the future. It's a powerful tool, but it's not like flipping a switch. Vaccines don't guarantee immediate and lifelong immunity for everyone who gets them, and the same is true for wild infection of a disease.

If the body isn’t exposed to the pathogen or vaccine again for a long time, the body can “forget” how to make the antibodies, and isn’t as able to adequately fight off an infection—even though the person was vaccinated. “Booster” shots can help keep the immune system primed and ready in case you come into contact with a wild form of the disease, but who and how often you need another dose of vaccine can vary.

While some vaccines provide seemingly lifelong immunity, others' protection fades over time, and—as is the case with the measles vaccine—not everyone is going to get a strong immune response. That means a certainly proportion of a given population will be vulnerable, even if vaccination rates are high.

In the case of pertussis specifically, there is also some evidence that vaccinated individuals get protection from active illness—but not necessarily from colonization. Put simply, if a vaccinated person comes in contact with the bacteria, they might not have a cough or fever, but they could still spread the bacteria to others through their respiratory droplets—like through kisses, for example. Researchers, though, are still looking into it.

It's important to note that while vaccines aren't perfect, they are still the best way to prevent diseases like mumps and pertussis.

Drug Resistance

Antibiotics used to be a magic bullet to cure a wide range of diseases. The discovery of penicillin in the late 1920s was a game-changer for mankind, as diseases that used to mean certain death suddenly became treatable. But just as humans have found ways to stave off disease, viruses and bacteria have been adapting, too.

Tuberculosis, for example, used to kill roughly one out of every seven people who got it. Effective diagnosis and treatment have led to a drop in incidence rates in the United States and globally, but that progress is threatened as drug-resistant tuberculosis continues to crop up around the world. In some instances, the bacteria appear to be incurable with existing programs and medications.

And it's not the only one. Drug resistance has been seen with a number of diseases—some of them posing urgent threats to public health, including sexually transmitted diseases like gonorrhea. The reasons behind how resistance develops varies, but it all comes down to how and when these drugs are used.

When you have a bacterial infection, it's possible that you already have some bacteria that are naturally immune to the antibiotic inside you, as well as some "good" bacteria that help protect your body from the "bad" bacteria. Antibiotics kill both, but when not used correctly—not finishing all of the prescribed amount, for example—they can leave some of those bad, resistant bacteria behind. Without good bacteria to keep them at bay, these "superbugs" can multiply, take over, and potentially spread from person to person or pass on their super-strength to other bacteria.

One of the most important steps in combatting drug resistance is to change the way antibiotics are used and prescribed. According to the Centers for Disease Control and Prevention, as much as 50 percent of the time antibiotics are prescribed, they are prescribed incorrectly or in a way that's suboptimal—for example, prescribing antibiotics for what is really a viral infection, like the cold.

Misuse of antibiotics in food-producing animals can also lead to drug resistance of food-borne illnesses like salmonella in humans, and thus should only be used under the supervision and direction of a licensed veterinarian. Individuals can also do more to prevent drug resistance by working harder to prevent illness in general through better hand-washing, safe food prep, and using medications only when necessary and as prescribed.

Climate Change

Perhaps the largest resurgence of disease is yet to come. With the rise in global temperatures, the earth is seeing changes in not only the environment, but also shifts in animal habitats and human interaction as extreme weather events—always a threat to human health and safety—become more frequent.

Scientists are cautioning that a warmer, wetter planet will lead to a resurgence of a number of diseases. Heavy rains and subsequent flooding, for example, can overwhelm drains and back up sewer lines, leading to the contamination of water supplies and outbreaks of diseases like cholera. Warmer temperatures and increases in precipitation are allowing tropical mosquito populations to creep ever-closer to the poles, risking a surge in vector-borne illnesses like malaria. And rising sea levels will likely displace whole communities and force them to move to increasingly urban spaces, where diseases can more easily spread.

When and where these events will happen is—at this point—largely theoretical due to the highly complex nature of disease transmission pathways. But health officials predict that the changing climate will likely, at the very least, exacerbate and expand current health issues, especially in areas that lack the infrastructure and resources to prepare and respond.

Flickers of that are already starting to appear. Dengue fever incidence has increased significantly in the past several decades, in part because of warmer temperatures and higher precipitation allowing its vector, the Aedes mosquito, to expand its habit. Reported cases of waterborne diarrheal diseases commonly seen following heavy precipitation—like legionella and cryptosporidium—have seen a rise in recent years, and warmer waters have made cholera-causing bacteria able to survive in areas they couldn't before. These increases might be only the beginning.

A Word From Verywell

The ebb and flow of disease incidence are highly complex and almost never due to a single, solitary cause. Examples given above are meant to illustrate how these specific factors are influencing disease trends, and aren't meant to represent an exhaustive explanation for why a particular disease is making a comeback. 

Additionally, while some of these pathogens are, indeed, showing signs of a resurgence, many more are conquered every day through large-scale and coordinated public health efforts. The significance of this success shouldn't be overlooked. 

Sources: 

Centers for Disease Control and Prevention. About Antimicrobial Resistance

Dayan GH, Rubin S, Plotkin S. Mumps Outbreaks in Vaccinated Populations: Are Available Mumps Vaccines Effective Enough to Prevent Outbreaks? Clin Infect Dis. 2008;47(11):1458-1467.

Phadke VK, Bednarczyk RA, Salmon DA, Omer SB. Association Between Vaccine Refusal and Vaccine-Preventable Diseases in the United States: A Review of Measles and PertussisJAMA. 2016;315(11):1149–1158.

Warfel JM, Zimmerman LI, Merkel TJ. Acellular pertussis vaccines protect against disease but fail to prevent infection and transmission in a nonhuman primate model. Proceedings of the National Academy of Sciences of the United States of America. 2014;111(2):787-792. 

World Health Organization. Climate and Health Country Profiles - 2015: A Global Overview.  

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