Anti-vaxxers have been named one of the top threats to global health in 2019 by the World Health Organization (WHO).

The anti-vaccine movement joined air pollution and climate change, HIV, and a worldwide influenza pandemic on the list released on Monday.

'Vaccine hesitancy', as the WHO calls it, 'threatens to reverse progress made in tackling vaccine-preventable diseases.'

The organization added in its statement: 'Vaccination is one of the most cost-effective ways of avoiding disease - it currently prevents [two to three] million deaths a year, and a further 1.5 million could be avoided if global coverage of vaccinations improved.'

Anti-vaxxers were named one of the top threats to global health in 2019 in a list released on Monday by the World Health Organization (file image)

A report released last year from the Centers for Disease Control and Prevention (CDC) found that the number of unvaccinated children up to 35 months old increased four-fold between 2001 and 2015.

There are several reasons why people are reluctant or refused to be immunized despite readily available vaccines.

A vaccine advisory group to the WHO listed some of the reasons as complacency, difficulty accessing vaccines and lack of confidence.

There are 18 US states that allow non-medical vaccine exemptions due to 'conscientious objector' or 'philosophical/personal beliefs'.

A survey from May 2018 found that support for vaccinations among Americans has fallen 10 percent in the last 10 years.

About 70 percent said common vaccines, such as for polio and measles, are 'very important', found the poll from Research America and the American Society for Microbiology.

This is down from 80 percent who gave the same answer in November 2008.

According to the CDC, more than 90 percent of children under age three have been vaccinated for polio, MMR (measles, mumps, rubella), Hepatitis B and chicken pox.

And more than 80 percent have received Haemophilus influenzae, diphtheria, tetanus, pertussis and pneumococcal infection vaccines.

However, mounting distrust has led some parents to not immunize their children, in turn leading to outbreaks of diseases not seen in years, such as measles, whooping cough and mumps.

Some anti-vaxxers avoid shots as a protest against big pharmaceutical companies, but some say vaccines are made from unnatural and unsafe chemicals and that they would rather take their chances on their kids getting treated if or when they get sick.

Others argue that vaccines overload a child's immune system or that natural immunity is better.

Then, of course, there is the argument that vaccines are linked to autism, a claim that has been debunked by scientists.

Experts say that, as diseases have become less common, people don't remember a time from before vaccines were commonplace.

'There are infections we haven't seen in years or we can't remember the last time we saw them,' Dr Michael Angarone, an assistant professor of medicine at Northwestern University Feinberg School of Medicine in Chicago, told DailyMail.com in an interview last year.

'So they ask: "Why should I vaccinate myself or my child if the disease is not around?" Well, then we'll start seeing more cases of measles, mumps, and polio again.'

According to the WHO, measles - a highly contagious but easily preventable disease - has seen a 30 percent increase in cases around the world.

Between September 2017 and August 2018, WHO reported more than 41,000 cases with 40 deaths in EU member states.

And, according to the CDC, 349 cases of measles were reported in 26 US states and Washington, DC.

It is the second-greatest number since measles was considered eradicated in the US in 2000.

The WHO said that, this year, it plans to ramp up efforts to eliminate cervical cancer by making the HPV vaccine more widely available and as well as providing more vaccines within Afghanistan and Pakistan.

The two central Asian countries were the only nations in 2018 where cases of wild poliovirus were confirmed - largely due to poor sanitation and low levels of vaccination coverage.