There is nothing more disheartening for a physician than watching a patient die from a preventable cause. And, of course, the loss for the family involved is unimaginable.

But it’s important, especially for parents, to understand the potential consequences of preventable, infectious diseases.

Here’s a scenario doctors across the country are witnessing first-hand: A 2-year-old girl develops what seems like a cold. Over the next several days, her breathing rate increases. At times, she stops breathing altogether for several seconds, followed by severe coughing spells and terrifying whooping sounds as she struggles to get air into her lungs. In spite of the best medical care, she experiences an uncomfortable and tragic death.

A century ago, this experience was common. Pertussis, commonly known as whooping cough, terrified parents and cost children around the world their lives.

Today, there are safe and effective vaccines to prevent these types of diseases in the first place. Yet a growing number of parents choose not to vaccinate their children, resulting in long-term disability and unnecessary deaths.

It does not have to be this way

When my father was a child, his sister died of measles. Her death stayed with him throughout his life. That was before we had a vaccine to prevent measles. If she had been born in the 21st century, she might not have died at age 6.

Outside of the U.S., approximately 1.5 million children die each year because their families can’t afford vaccines.

Here in the United States, we’re much more fortunate. Nearly all Americans today have access to a broad range of highly effective and safe vaccines. But a growing number of children aren’t getting them.

We need to ask ourselves this:  If economics are not the deterrent, why would a parent put their child’s life at risk by voluntarily foregoing a life-saving preventive measure? The answer is a combination of false science, outdated anecdotes and fear mongering.

The birth of vaccines

The concept of vaccination was pioneered by Edward Jenner more than two centuries ago.

In the late 1700s, Jenner discovered that individuals who had previously contracted cowpox, a disease with very mild effects on humans, were at little risk for becoming ill with smallpox, a disfiguring and often fatal disease.

He concluded that inoculating individuals with cowpox would prevent children from acquiring smallpox in the future. From that observation, the first vaccine was born.

Since Jenner’s discovery, many other vaccines have been developed, refined and introduced into clinical medicine. Many of these vaccines are mandated for children beginning school, including measles, polio and tetanus.

The global impact of these advancements is tremendous. The overall incidence of vaccine-preventable diseases declined dramatically during the 20th century. Smallpox has been eradicated worldwide. And thanks to the efforts of many groups, including the Bill and Melinda Gates Foundation, polio has been nearly eliminated.

Through the “Decades of Vaccines Collaboration,” 200 countries have endorsed a shared vision of a world where all individuals and communities enjoy lives free from vaccine-preventable diseases.

Extending the full benefits of immunization to every person worldwide by 2020 would prevent an estimated 20 million deaths — mostly in children — and untold suffering for millions more.

The rise of the anti-vaccine movement 

In the United States, we are witnessing the scientifically ignorant and sometimes deadly impact of an anti-vaccine movement. Individuals who support the movement continue to question the safety and necessity of vaccines despite extensive medical literature to the contrary.

When laboratory-produced vaccines were first introduced over 50 years ago, there were legitimate concerns about their safety. Many vaccines in their older forms were associated with the risk of rare but dangerous reactions.

The vaccines we use today have minimal risks and an extremely safe track record. They have undergone rigorous testing and scrutiny by the scientific community and have proven their effectiveness in large-scale clinical trials.

As a result, the days of school closures for measles and pertussis outbreaks have become a relic of the past. The side effects from vaccines are almost always mild. And even in the extremely rare case of a more serious allergic reaction, physicians and their staff are trained to deal with it.

Simply put, the benefits of vaccination substantially outweigh the risks.

Yet for the last two decades, fear mongers associated with the anti-vaccine movement in the U.S. and other developed countries have convinced some parents to refuse to vaccinate their kids.

The result is an erosion in health gains, both individual and collective. And in some parts of the country, we are witnessing a reversal of what many believe is one of the greatest advances in medical science in the last century.

The fear mongering behind measles and whooping cough

Measles and whooping cough are very serious, highly contagious respiratory diseases spread through the air by breathing, coughing or sneezing.

Although their clinical symptoms are different, both carry risks of long-term problems and even death.

Measles begins with fever, runny nose, cough and a rash all over the body. Before the introduction of a measles vaccine in 1963, hundreds of thousands of people in the U.S. contracted the disease annually. Thousands were permanently disabled and between 400 and 500 people died. But since 1963, reported cases fell to less than a thousand a year.

Things started changing in 1998 when a British physician published a study in the Lancet medical journal that falsely asserted a connection between autism and the combined measles-mumps-rubella (MMR) vaccine.

An investigation into the work revealed the research was unethical and rife with conflicts of interest. The article was filled with false and fraudulent data, and the health care risks described have been completely discredited. In 2010, the paper was fully retracted from the Lancet, a remarkable event in the world of peer-reviewed journals.

But the damage was done. Vaccination rates in the UK plummeted and reported cases of measles soared. In the U.S., new measles cases have tripled as of 2013, with reported outbreaks in eight American communities. The recent outbreak in New York City has sickened at least a dozen people.

Meanwhile, whooping cough, a highly contagious bacterial infection, has seen a huge increase in the number of people infected each year.

The incidence of whooping cough was relatively low in the U.S. — around 5,000 cases annually — when vaccination was the unchallenged standard of care. But the impact of the anti-vaccine rhetoric and associated fear has contributed to several outbreaks across the United States and Europe, resulting in multiple infant deaths.

In 2010, three were 9,000 cases of whooping cough reported in California alone, causing the deaths of 10 infants under the age of 1 – the most in the state since 1947.

The first whooping cough vaccine was developed in the mid-1920s.  By the mid-1940s, it was used widely and often administered in combination with the diphtheria and tetanus vaccines.

In 1991, a combination vaccine called DTaP reduced the frequency of side effects and eliminated nearly all major adverse reactions from whooping cough immunization.

Unfortunately, California is now one of 19 states that allow “personal belief” exemptions for parents before their children enter school. As a result, non-medical exemptions in California have tripled between 2000 and 2010 with some schools in affluent communities reporting rates as high as 84 percent.

And as the 2010 outbreak demonstrated, clusters of whooping cough appear most frequently in these communities with higher than average non-medical exemptions.

Even if this exemption did not exist, there will always be some individuals who will not be vaccinated and others who will lose their immunity decades after the vaccine is given. Protecting these folks requires what health experts call “herd immunity.”

If a single parent does not immunize a child, the risk to that individual is low. But as the number of unvaccinated children grows, the risk of numerous people contracting and spreading the disease multiplies, creating a public health risk for a large segment of the population.

For highly contagious diseases like whooping cough and measles, herd immunity is dependent on having 95 percent of the population in a community immunized. When the immunization rate falls, the danger to both the young and elderly increases dramatically.

A plea to parents

We have highly safe and effective vaccines readily available to prevent many of the most dangerous childhood diseases. Yet despite decades of research that demonstrate their overwhelming positive impact on the health of our children, we are losing ground.

Before parents decide not to vaccinate their son or daughter, they need to consider the scientific evidence. They need to imagine how they will feel should their child die or experience long-term disability from an easily preventable disease.

And as a society, before we allow misinformation to threaten public health, we must recognize that vaccines today are safe and effective. Anything less is irresponsible. We owe it to our children and our communities to make vaccination universal.

Robert Pearl is a physician and CEO, The Permanente Medical Group. This article originally appeared on Forbes.com.