Five People Whose Discoveries Saved Millions of Lives

A dying 14-year-old boy became part of a medical breakthrough that would change the lives of millions. On January 11, 1922, Leonard Thompson received an experimental injection at Toronto General Hospital while weighing just 65 pounds and fighting a disease that had almost certainly doomed him.

The first attempt failed, leaving him with an abscess from a crude pancreatic extract. Twelve days later, doctors tried again with a purified version, and his blood sugar dropped to normal levels within a day.

A Boy’s Fight Changed Diabetes Forever

In 1922, a diagnosis of diabetes often had only one ending. Doctors had no reliable way to replace the function of the insulin-producing cells that had failed inside the body.

Frederick Banting and Charles Best had developed an early pancreatic extract, but it still needed refinement before it could become a safe treatment. James Collip, a biochemist working with the team, helped purify the substance that was eventually given to Thompson.

Insulin Became a Global Lifeline

The breakthrough transformed diabetes from a rapidly fatal illness into a condition that could be managed. BBC Science Focus has credited the combined insulin breakthrough with saving more than 200 million lives, although no estimate of that scale can be counted with complete precision.

The researchers also made an extraordinary decision about ownership. On January 1, 1923, the patent was assigned to the University of Toronto’s Board of Governors for the sum of one dollar, helping ensure the discovery would not simply become a private fortune.

A Forgotten Petri Dish Changed Medicine

Alexander Fleming’s most famous discovery began with something that looked like a laboratory mistake. In 1928, he returned from holiday and found mould growing on one of his forgotten Petri dishes.

Around the mould was a clear area where bacteria had stopped growing. Fleming had discovered the antibacterial properties that would eventually lead to penicillin.

Fleming Did Not Work Alone

Finding the mould was only the beginning of the story. Howard Florey and Ernst Chain later spent years helping turn Fleming’s observation into a purified medicine that could be produced and used to treat patients.

The three men shared the Nobel Prize in Physiology or Medicine in 1945. The list in the source article credits the penicillin breakthrough with saving more than 200 million lives, placing its estimated impact alongside insulin.

Fleming also warned about the danger of bacteria becoming resistant to antibiotics. In his Nobel lecture, he cautioned that exposing microbes to insufficient amounts of penicillin could help them survive and adapt, a problem that remains central to modern medicine.

Maurice Hilleman Created Vaccines at Scale

Maurice Hilleman’s name may be unfamiliar to many people, but his work remains part of everyday public health. Born in Montana in 1919, he spent his career developing more than 40 vaccines.

Eight of those vaccines remain on the American childhood immunization schedule. They protect against diseases including measles, mumps, hepatitis A, hepatitis B, chickenpox, meningococcal disease, pneumococcal disease and Hib disease.

His Work Still Saves Lives Today

In 1957, Hilleman recognized the potential for a flu pandemic and helped accelerate vaccine development in time to respond to the outbreak. The source list credits him with saving more than 130 million lives, while the Children’s Hospital of Philadelphia has said his work saves about 8 million lives every year.

That estimate is striking because Hilleman died in 2005. His research continues to affect children and families decades later through vaccines that remain part of modern healthcare.

His place on the list also raises an interesting question about fame. Some of the people who have shaped human survival most dramatically are far less recognizable than celebrities, politicians or even other scientists.

Norman Borlaug Took His Work to the Fields

Medicine is not the only way to keep people alive. Norman Borlaug spent much of the 1950s and 1960s developing wheat varieties that resisted disease and produced higher yields.

His work in Mexico became closely connected to the Green Revolution, a period of major agricultural change that introduced new crop varieties and farming methods in several parts of the world. Borlaug received the Nobel Peace Prize in 1970 for his efforts to address hunger.

The Numbers Attached to Him Are Enormous

The list cited in the source article credits Borlaug with saving 245 million lives. Other accounts have attached a much larger estimate to his name, claiming that his work helped save as many as one billion people.

Those two figures reveal a major problem with rankings like this. There is no universal method for calculating how many people would have died in a world where a particular discovery or agricultural development had never happened.

Research professor Glenn Davis Stone has also challenged some of the broad claims surrounding the Green Revolution in India. His analysis argued that food grain production did not accelerate during the period in the way popular accounts often suggest, while dependence on fertilizer increased significantly.

The Arithmetic Behind “Lives Saved” Is Uncertain

The people on this list changed conditions that affected enormous populations, but counting the exact number of lives saved is almost impossible. Nobody can create a complete record of people who did not die because insulin, antibiotics, vaccines or improved crops became available.

That does not mean the discoveries were unimportant or imaginary. It means the massive totals attached to famous names should be read as estimates rather than precise measurements.

Several factors make those calculations difficult:

• Alternative outcomes: Researchers must estimate what might have happened without a discovery.
• Shared credit: Major breakthroughs often involve teams working across decades.
• Population changes: A treatment may affect different numbers of people over time.
• Indirect benefits: One discovery can influence later medical or technological advances.

The estimates are useful for showing scale, but they can create an illusion of mathematical certainty. A number such as 245 million sounds exact even when the historical reality behind it is much more complicated.

Blood Groups Made Transfusions Safer

Before the beginning of the 20th century, blood transfusions were unpredictable and dangerous. Doctors could transfer blood from one person to another without understanding why some patients survived while others suffered severe reactions.

Karl Landsteiner helped solve that problem when he identified the major blood groups A, B and O in 1901. His discovery made it possible to understand how blood compatibility affected the safety of transfusions.

Blood Also Had to Be Preserved

Matching blood types was only part of the challenge. Doctors also needed a way to stop blood from clotting before it could be stored and used.

In 1914, Belgian physician Albert Hustin used sodium citrate to prevent blood from clotting outside the body. The source notes that several doctors independently developed similar approaches around the same period.

Together, Landsteiner and Hustin are credited with saving more than one billion lives, the largest estimate on the list. Modern surgery, trauma treatment and emergency medicine would be almost impossible to imagine without systems built on the discoveries that made blood transfusions safer.

Some Major Life-Savers Never Make the List

Any attempt to name the five people who saved the most lives will leave out obvious candidates. Edward Jenner, whose work became a foundation for vaccination, does not appear on this particular list.

Jenner carried out his famous cowpox experiment in 1796, helping establish an approach that eventually contributed to the global fight against smallpox. The World Health Organization declared smallpox eradicated in 1980, making it the only human infectious disease to have been wiped out.

Progress Rarely Belongs to One Person

Clean water systems, sewage networks and chlorinated water supplies have also transformed human survival. Their impact has been enormous, yet no single person can easily claim ownership of those developments.

That is one of the limits of ranking history through individual faces. Science and public health often advance through the combined work of researchers, doctors, engineers, institutions and ordinary people who never become household names.

The five people in this article are a defensible selection, but they are not the only possible answer. The history of human survival is too large and too collaborative to fit neatly into a single ranking.

Leonard Thompson Received Thirteen More Years

The biggest number attached to this story may be more than one billion, but the most personal number is 13. Leonard Thompson lived for another 13 years after receiving insulin and died in 1935 at the age of 27.

Those years would have been difficult to measure on any list of humanity’s greatest achievements. Yet they show what the enormous estimates actually represent: individual lives continuing when they otherwise might have ended.

Every calculation about millions or billions eventually comes back to people who received more time. A hospital patient went home, a child survived an infection, a family had food, or someone reached an age they were never expected to see.