BSIP/UIG/Getty

The World Health Organization has released a list of the biggest threats to global health in 2019 and, perhaps unsurprisingly, air pollution and climate change top the list.

Read more: How to track the 2019 measles outbreak

But it is another threat -- "vaccine hesitancy" -- that the WHO has named as a major health problem and it has garnered a lot of attention, with the organization suggesting the reluctance or refusal to vaccinate may "reverse progress made in tackling vaccine-preventable diseases", such as measles and diphtheria.

Measles cases spiked in 2018 due to a "gaps in vaccination coverage", according to the WHO. The disease is highly contagious and spread through the air and may cause a rash, fever and even inflammation of the brain. It can be particularly dangerous to young children and fatal. In 2016, the Americas were declared free of the disease, but it has re-emerged in the past two years, with The New York Times reporting on Jan. 17 that New York State is just coming out of "its most severe outbreak" of measles in decades.

Similarly, diphtheria cases are on the rise. In 2018, diphtheria made the WHO's list on its own because it had made "an alarming comeback" due to "significant gaps in healthcare provision". In Venezuela, cases rose from 31 in 2016, to 786 in 2017. India saw an increase from 3,380 cases to 5,293 in the same time span.

Both diseases are readily preventable with vaccinations, but their rise in the last three years has seen the WHO take the anti-vaccination movement a little more head-on.

"Vaccination is one of the most cost-effective ways of avoiding disease," the WHO writes. "It currently prevents 2-3 million deaths a year, and a further 1.5 million could be avoided if global coverage of vaccinations improved."

In adding vaccine hesitancy to the threats list, it joins the Ebola virus, fragile and vulnerable settings, HIV, Dengue fever, noncommunicable diseases like diabetes and heart conditions, weak primary health care and antimicrobial resistance as some of the world's most pressing health problems in the coming year.

Air pollution is considered the "greatest environmental risk to health" in 2019. According to the WHO, "nine out of ten people breath polluted air every day" and our insistence on burning fossil fuels not only contributes to that statistic but is the major driver of climate change. Because climate change has such wide and varied effects on the environment, it contributes to global health issues in a number of ways -- driving the incidence of tropical disease up and increasing health complications via heat stress and malnutrition.

The WHO releases an annual list of global health threats at the beginning of each year, but notes 2019 sees the start of a new 5-year stategic plan that focuses on "a triple billion target". That target is part of the WHO's wishes to ensure that 1 billion more people are protected from health emergencies, enjoy better health and can access universal health coverage.

Taking It to Extremes: Mix insane situations -- erupting volcanoes, nuclear meltdowns, 30-foot waves -- with everyday tech. Here's what happens.

Fight the Power: Take a look at who's transforming the way we think about energy.