Skepticism about the safety of childhood vaccines is a sentiment shared by a growing number of politicians and parents, who are choosing to skip recommended shots for their children.

But while everyone seems to be talking about the potential side effects of vaccines, few are discussing the diseases they prevent.

It has been half a century or more since many of the inoculations became routine in the United States, and the experience of having these illnesses has been largely erased from public memory.

Here is what people should know about six once-common illnesses that vaccines have contained for decades.

Measles

Measles, a viral infection often spread by a cough or sneeze, is extraordinarily contagious: Nine of 10 people around an infected person will catch measles if they have not been vaccinated. Measles can be contracted in a room up to two hours after a person with the disease has left it.

Measles is not a mild illness, particularly for children younger than 5. It can cause a high fever, coughing, conjunctivitis and rashes, and if it leads to pneumonia or encephalitis — brain swelling — it can quickly become lethal. Before the vaccine was licensed in the United States in 1963, almost every child had contracted measles by age 15. Tens of thousands of measles patients were hospitalized each year, and between 400 and 500 of them died.

Two doses of the MMR vaccine together are about 97% effective at preventing measles. About 280,000 kindergarten students in the United States are now unprotected, according to the Centers for Disease Control and Prevention.

Diphtheria

The bacterial infection that creates a thick, gray membrane over the throat and tonsils, suffocating its victims. There was a time in the United States when up to eight children in a single family suffered that fate — a burden so grave that a science historian called it “childhood’s deadly scourge.”

The toxin driving the disease is produced by a strain of bacteria in respiratory droplets and works by killing healthy tissues, which can lead to difficulty breathing and swallowing, especially among young children with smaller airways. It can also gravely damage the cardiac and nervous systems, resulting in heart failure or paralysis.

Even with treatment, 1 in 10 people who have respiratory diphtheria die from it, according to the CDC.

Tetanus

A fully developed tetanus infection can be an alarming sight: fists clenched, back arched, legs rigid from extreme, excruciating muscle spasms that last several minutes. Extreme fluctuations in blood pressure. A racing heart. Neck and stomach muscles tight enough to impair breathing.

Treatment for tetanus must be immediate, and up to 20% of people who become infected will die.

It all starts with a bacteria that lies dormant in soil and animal feces until it enters the body through broken skin like a cut. Children are now protected through multiple doses of the dTap vaccine, which also guards against diphtheria and pertussis (also known as whooping cough).

Mumps

The mumps virus, spread through saliva and respiratory droplets triggers a fever and swollen salivary glands in the ears — which is why patients often have a puffy jaw and cheeks — and can, in severe cases, cause deafness.

The disease is dangerously insidious: It can lie dormant for up to a month before symptoms appear, and most people are infectious before their salivary glands begin to swell. Complications are more common in adults than children, but they can include inflammation in the ovaries and testicles — which can cause infertility or sterility — or in the brain and spinal cord, which can put patients at risk of seizures and strokes. Annual cases in the United States — which previously hovered between 200 and 400 — have surpassed 1,000 nine times since 2006.

Rubella

The first sign of rubella is often a rash on the face, and while the infection often remains mild in children, it can prove devastating for pregnant women whom the children infect.

When passed on to a fetus, rubella can cause a miscarriage or lead to severe birth defects, such as heart problems, liver or spleen damage, blindness, and intellectual disability. At least 32,000 babies worldwide are born annually with congenital rubella syndrome. About a third of them die before their first birthdays.

Rubella is transmitted through coughing and sneezing, and up to half of people who spread the infection do not know they have it. Most women who contract rubella in adulthood say they experience arthritis. In rare cases, rubella can also cause brain infections and bleeding problems. There is no specific treatment.

Polio

Parents in the early 1950s lived with a terror few could later imagine: the substantial prospect that their child could touch the wrong toy and end up in a wheelchair, an iron lung or a grave.

Polio epidemics, which had been occurring for decades, had gained new magnitude by the middle of the 20th century, killing or paralyzing more than 500,000 people worldwide each year. Families were avoiding public spaces and turning down summertime play dates, knowing that the malady struck fast: In the words of historian and author Richard Rhodes, “One day you had a headache and an hour later you were paralyzed.”

In some parts of the world, the disease is still a major threat. It is transmitted by exposure to fecal matter, such as on contaminated foods or objects. Most people who contract the virus have no visible symptoms, though they can still pass it on. About a quarter develop common flu symptoms such as a sore throat, fever and nausea.