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
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.
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.
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.
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.
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.
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.
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