This year marks 70 years since the first polio vaccine was declared safe and effective, and more than 45 years since the disease was wiped out in the United States. A large part of that story starts in Pittsburgh.

In 1947, Dr. Jonas Salk, then a 33-year-old physician and researcher, accepted a professorship at the University of Pittsburgh Medical School. The next year he started looking into different strains of the polio virus and, by 1955, the vaccine he'd created was going into public use.

Poliovirus, which causes polio, is extremely contagious, and can be transmitted by poor sanitation or by contact with an infected person's saliva or fecal matter.

It's also been with us a long time. There are depictions in ancient Egyptian art showing people with the atrophied limbs characteristic of paralytic polio.

A recent paper explained that polio can present in many ways.

"The presentation of polio is highly variable, ranging from asymptomatic to a transient flu-like viral illness to paralysis, quadriplegia, and even respiratory failure and death," according to the paper by Jonathan G. Wolbert, Michael Rajnik, Helena M. Swinkels and Karla Higginbotham.

In most cases, people infected with poliovirus have mild flu-like symptoms, but occasionally, the virus spreads to the central nervous system.

However, in about 0.5% to 1% of polio infections the worst case scenario hits. In those cases, known as paralytic polio, the virus affects nerve fibers, and can cause paralysis of a single limb or multiple limbs, as well as the chest and abdomen.

The U.S. Centers of Disease Control and Prevention reports that in about 20% of paralytic polio cases, the virus affects the nerves of both the spinal cord and the bulbar region of the brain stem. That can leave patients unable to breathe on their own, requiring a the use of an irong lung.

For most of human history, polio was a childhood disease. And while it can be dangerous to children, the effects are far more dire on adults.

"The case fatality ratio for paralytic polio is generally 2% to 5% among children and up to 15% to 30% among adolescents and adults," according to the CDC. "It increases to 25% to 75% with bulbar involvement."

While polio has been infecting humans for thousands or possibly event tens of thousands of years, epidemics were essentially unknown until the age of industrialization. By the 1900s, outbreaks were happening regularly throughout Europe and the United States.

There are a number of theories about why that happened, but the most widely accepted is that increased sanitation and hygiene limited exposure to the virus during infancy and early childhood, when the disease is less likely to cause paralysis.

The authors of "The Spatial Dynamics of Poliomyelitis in the United States: From Epidemic Emergence to Vaccine-Induced Retreat, 1910–1971," write that the theory explains why older people were infected.

"The hygiene model (accounts) for the gradual development of more sizeable epidemics of poliomyelitis, associated with increasingly older patient cohorts, as the 20th century progressed," they wrote.

In the same paper, the authors show exactly how bad the situation was by the 1950s.

"At its height, from 1950–1954, poliomyelitis resulted in the paralysis of some 22,000 U.S. citizens each year, equivalent to an average annual rate of 14.6 per 100,000," they wrote. "Many thousands were left permanently disabled by the disease, while many others suffocated as a consequence of respiratory paralysis."

Salk was born in 1914, just as the disease was starting to grip the U.S.

In his 1990 memoir, "A Hole in the World: An American Boyhood," journalist and author Richard Rhodes, wrote about the atmosphere of the time.

"Polio was a plague," Rhodes wrote. "One day you had a headache and an hour later you were paralyzed. How far the virus crept up your spine determined whether you could walk afterward or even breathe. Parents waited fearfully every summer to see if it would strike. One case turned up and then another. The count began to climb."

Salk graduated from New York University School of Medicine in 1939, but he never wanted to be a working physician.

"My intention was to go to medical school, and then become a medical scientist," Salk said in an interview with the Academy of Achievement. "I did not intend to practice medicine, although in medical school, and in my internship, I did all the things that were necessary to qualify me in that regard. I had opportunities along the way to drop the idea of medicine and go into science.

“At one point at the end of my first year of medical school, I received an opportunity to spend a year in research and teaching in biochemistry, which I did. And at the end of that year, I was told that I could, if I wished, switch and get a Ph.D. in biochemistry, but my preference was to stay with medicine.

“And, I believe that this is all linked to my original ambition, or desire, which was to be of some help to humankind, so to speak, in a larger sense than just on a one-to-one basis."

After graduating, Salk spent several years studying viruses and in 1947 he accepted a position as a professor at the University of Pittsburgh School of Medicine. The next year he started research into poliovirus.

For the next four years, Salk and his colleagues in the lab developed a vaccine using inactivated poliovirus.

In July 1952, after testing on animals, Salk injected his vaccine into children who already had polio. The children produced more antibodies, showing the vaccine likely worked.

Next, Salk followed a long scientific tradition and vaccinated himself, along with his wife and children and other volunteers. When none of them got sick and they, too, started producing antibodies, he knew it worked.

That same year saw the worst polio outbreak in U.S. history, with more than 57,000 cases, more than 3,100 deaths and more than 21,000 left with some degree of paralysis, according to U.S. public health figures. And 1953 saw more than 35,000 cases.

In March 1953, Salk spoke to CBS News and announced the results of the early trials and published his results in the Journal of the American Medical Association.

In 1954, 1 million children would be vaccinated in a large-scale field test. Among the very first were the students at what's now the Watson Institute in Sewickley, in Allegheny County.

By September 1954, the Butler Eagle ran a story saying none of the children in Pennsylvania who’d been vaccinated earlier that year were infected with polio.

Finally, in April 1955, the vaccine was declared safe.

Almost immediately, cases plummeted. In 1957, two years after the start of mass vaccination efforts, there were about 5,600 cases, about 10% of the number five years before.

In 1961, a new vaccine, one taken orally instead of injected, was launched. It used live but weakened poliovirus.

That same year, U.S. cases fell to about 160, according to CDC figures. The last case of polio transmitted in the U.S. happened in Amish communities in the Midwest.

By 1994, the Americas were free of polio, according to figures from the World Health Organization. Global polio eradication efforts have continued, and as of 2024 there were only two countries in the world — Afghanistan and Pakistan — where polio is still endemic. Those two countries saw a total of 12 cases in 2023 and 93 cases in 2024.