Open Source CEO by Bill Kerr- Posts
- The Black Death & How It Birthed Today's Prosperity
How one little flea, on one little rat, changed the course of history. 🦠 #
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So, Midjourney, the company that brought us AI image generation, just announced Midjourney Medical and Midjourney Spa. Sounds weird, right? Well, what’s even crazier is they claim to have developed a new “full body ultrasound” technique that produces whole-body imagery in 60 seconds. “There is no radiation, no powerful magnetic fields—just sound and water and 60 seconds.”
Source: Midjourney’s release announcement. And what’s even more audacious is their goal of deploying “around 50,000 of these scanners around the world over the next 6 years and use this fleet of sensors to do a billion full-body scans every month.” Certainly not what I had on my bingo card for June 18th, 2026. Humans are awesome. Enjoy today’s piece.
BUSINESS STORY** 🗞️**
Every so often since the dawn of time, an event happens that meaningfully changes the course of history. From the Big Bang to the killing of the dinosaurs, and the fall of the Roman Empire. If the butterfly effect can claim that the fluttering of a pair of micro wings can set off a meaningful chain reaction, then these course corrections in our history are that, multiplied by an order of magnitude or two.
Another such event in our past is something so gruesome that the mere thought of it can send shudders down your spine. An occurrence that wiped a large portion of our population right off the face of the Earth. An airborne disease that saw blood and pus seep from the body, followed by fever, chills, vomiting, diarrhea, terrible aches and pains. And then of course, death. I am referring to the GOAT of the infectious disease game, the King of the Killing, the Muhammad Ali of existence expunction, the one and only Bubonic Plague. The Black Death.
But as I have already alluded to in the title of this piece, there were profound, second-order effects that gave rise to the prosperity that we all share today. And my aim today is to show you how. So kick up your feet, grab a warm glass of medieval mead and come with me on a dark, shadowy romp through the grim annals of the original pestilence. And the economic factors it gave rise to that we appreciate today.
If I had a nickel for the number of times I have heard my friends say, “Oh man, I wish I lived back in the Middle Ages; life was so much better back then”…I’d be dead broke. No one says that, and if by chance they did, they’ve clearly never read a single paragraph of actual history. Marauding armies would routinely pillage your village; consistent famines saw people eating bark, roots, and even resorting to cannibalism, and don’t even get me started on the quality of life for the average criminal. Steal a loaf of bread after a few too many meads, and you were likely to be burned alive, drawn and quartered, or have your entire body broken on the wheel.
No wonder life expectancy was around 30. Most of life was just waiting to die from plague, childbirth, or infected splinters. But sure. No emails. What a vibe.
The economic situation was not much better. You lived in a fixed social hierarchy known as feudalism. This meant you had stagnant wages, for well, ever. And for the average person, you probably lived as a serf, which is a step up in standard of living and human rights from a slave. But only a very small step.
*I am picturing those tiny little dachshund ramps. We have one, for our tiny little family member.
To dive a little deeper into serfdom; to be a serf meant that you were essentially a peasant that was bound to a piece of land, and therefore also bound to the will of the lord. Lords, as we know from popular culture, could often be quite malevolent in nature; the Sheriff of Nottingham and the like. It wasn’t slavery, but it also wasn’t peasantry. The peasants of the 1300s enjoyed the freedom to move around at their will, but would often be fighting for scraps in big cities. For this reason, estimates place the share of serfs in the European population at 50-75% during this time.
There was no democracy. No civil rights, and certainly no concept of something as fancy as ‘freedom of speech.’ You were born into a class—quite likely a very low one—and mostly stayed there.
Life on the higher rungs of society was much better but still lacked basic functions we take for granted today, like, for one, medicine. Hospitals didn’t exist; maybe you’d head down to the monastery for some bloodletting, of course. ‘Germs’ as we know them were not even a twinkle in the most tin-hat conspiracy theorists' eyes, and vaccines, antibiotics, and anesthesia were hundreds of years away.
A time when potions ruled the world.
As it was, the leading framework for healthcare in Europe for over 1,000 years, from Ancient Greece right through to the 1600s, was the Four Humors. Although it wasn’t until the scientific revolution (1600s–1700s) + microscopes + germ theory that it finally died, people were still using bloodletting until the 1800s.
| Humor | Organ | Temper | Season | Element |
|---|---|---|---|---|
| Black bile | Spleen | Melancholy | Cold and dry | Earth |
| Yellow bile | Lungs | Phlegmatic | Cold and wet | Water |
| Phlegm | The head | Sanguine | Warm and wet | Air |
| Blood | Gallbladder | Choleric | Warm and dry | Fire |
So that’s a little bit about social status and healthcare. But it doesn’t end there. Sanitation didn’t exist. Instead, you threw your number twos out the window. Education was more or less nonexistent. Johannes Gutenberg wouldn’t be born until the 1400s, meaning the printing press would be born in 1440. You got your news from gossip, church sermons, or a wandering bard. Oh, and if you could sign your own name, you were basically a genius.
If that all sounds bad, it was. But things were about to get much, much worse. 🔥** **Recommendation: Painfotainment by Dan Carlin’s Hardcore History is a dark look at punishment in the Middle Ages.
Let's follow a flea named—well, let's call him Death. Death lived quite comfortably in the fur of a black rat aboard a Genoese trading vessel in 1347. Said rat lived quite comfortably in the ship's grain stores. The ship was sailing from port to port across the Mediterranean. Death carried in his tiny little stomach something that would make medieval Europe's existing problems—the casual torture, the rampant disease, the 30-year life expectancy—look like a golden age of prosperity and wellness.
When Death's fat rat host died, our micro protagonist simply hopped onto the nearest warm body: a dock worker in the Sicilian port of Messina. That hop, no bigger than the width of your fingernail, would help kill one-third of Europe. Into this already nightmarish medieval world had come the ultimate nightmare.
If you are reading this, there is a strong chance that your family lived through this incredibly perilous period. If you have European heritage, then your ancestors were part of the lucky cohort. Imagine living through this period; watching entire families you knew be wiped out overnight, villages becoming ghost towns, mass graves and burning bodies. Monty Python familiarised us with the idea of carts rolling through the streets calling out to “Bring out your dead!” Source: |
The numbers tell a story of unimaginable devastation. In Florence, the population plummeted from 120,000 to just 50,000 in three years. Paris lost half its residents; 50,000 people simply gone. Hamburg and Bremen saw 60% mortality rates. Entire regions were depopulated so quickly that chroniclers ran out of ways to describe the carnage. One writer in London claimed only fourteen people survived in the entire city; almost certainly an exaggeration born of trauma, but it captures the psychological impact of watching civilization crumble around you.
The speed was perhaps most terrifying of all. Unlike famines or wars that might ravage a region over years or decades, the Black Death could empty a village in weeks. Symptoms appeared on Monday; you were dead by Friday. Families would go to sleep healthy and wake to find half their members developing the telltale buboes; painful, egg-sized swellings in the neck, armpits, and groin that signaled almost certain death.
This was what life looked like during The Black Death, the sickness that would wipe out between 30-60% of Europe in seven short years, with a total body count of 200M (and still rising technically).
It’s obvious how shocking this would have been to live through. But again, to make matters worse, on top of the loss of life, medieval society had to wrestle with agricultural collapse, trade-route disruption, and the failure of basic services. Fields were left unworked, and crops were allowed to rot, because, well, dead men plow no fields. Yet in this chaos, the seeds of a new world order were being planted.
What happened next would have shocked medieval observers. Rather than civilization collapsing entirely, the massive loss of life created something unprecedented: a seller's market for human labor. For the first time in centuries, ordinary people held the power to negotiate.
Adam Smith wouldn't be born for another 400 years, but his ‘invisible hand’ was already hard at work in plague-ravaged Europe. The Scottish economist would later write that, "The market price of every particular commodity is regulated by the proportion between the quantity which is actually brought to market, and the demand of those who are willing to pay." In 1350, Europe was about to witness the most extreme example of this principle in human history.
The supply was labor. The demand was survival. And suddenly, catastrophically, the supply had just been cut by half.
Smith would have predicted exactly what happened next. When supply falls short of demand, prices rise. But he also understood that individuals pursuing their own self-interest are "led by an invisible hand to promote an end which was no part of his intention." The surviving peasants weren't trying to overthrow feudalism—they just wanted better wages and working conditions. Yet their individual negotiations would accidentally demolish a social system that had endured for centuries.
[The] mortality destroyed more than a third of the men, women, and children … such a shortage of workers ensued that the humble turned up their noses at employment, and could scarcely be persuaded to serve the eminent unless for triple wages. … As a result, churchmen, knights and other worthies have been forced to thresh their corn, plough the land and perform every other unskilled task if they are to make their own bread.
As previously outlined, the life of a serf was grim at best prior to (and during) the Bubonic Plague, but the balance of power had shifted, and so were the fortunes of the medieval peasantry. Free agency of human capital had begun. | Fleabottom rising. |
This same function was beginning to happen in plague-ravaged Europe. Yes, technically serfs were required by law to take employment where their agreements deemed, but the lower ranks were flipping the proverbial bird to their lords and ladies, and taking to the road in numbers difficult to prosecute all over the continent. In Game of Thrones, the eunuch Varys quips that, “Power resides where men believe it resides,” and the peasant believed they had the power. And they’d be right.
The numbers tell the story of this seismic shift. In northwestern European cities, real wages didn't just rise; they exploded upward by 60-80% in the decades following the plague. A skilled craftsman who earned three grams of silver per day in 1340 was commanding 6-7 grams by 1380. For the first time in centuries, ordinary workers could afford meat regularly, not just on feast days.
But here's where it gets interesting: the economic transformation wasn't uniform across Europe. Central and southern cities initially saw wages crash during the plague years, amid widespread chaos and few functioning markets. Yet by 1380, even these regions had caught up, with wages doubling from pre-plague levels. The invisible hand had done its work, but geography mattered.
The conniving Lords tried everything to stop this economic revolution. England passed the Statute of Laborers in 1351, fixing wages at pre-plague levels and banning workers from leaving their home parishes. The French implemented similar measures. It was medieval price control at its most desperate, and about as effective as you'd expect. Workers simply ignored the laws, moving to wherever they could get better deals.
Source: Alfani (2020), VoxEU. Imagine if half of Amazon's warehouse workers disappeared overnight. Not quit—just vanish. Suddenly, the newly jacked Jeff Bezos would be personally delivery trucks. This is essentially what happened to medieval Europe.
Within months, Amazon would be forced to completely reimagine its business model. No more surveillance software tracking bathroom breaks. No more impossible productivity quotas. Instead, they'd be offering signing bonuses, stock options, and probably free Prime memberships for life. Workers would start demanding things unthinkable before the crisis: four-day work weeks, mental health benefits, actual chairs at packing stations.
We've actually seen a milder version of this recently. During the pandemic tech boom, software engineers became so scarce that companies offered absurd signing bonuses, remote work forever, and even coverage for egg freezing. When talent becomes scarce, companies get creative fast. It's basic supply and demand, and it's a beautiful thing. And something we should celebrate. It leads to less economic disparity and a rising quality of life for the masses.
All thanks to our little friendly flea we named Death earlier in our piece, the manorial system—the economic backbone of feudalism for 500 years—began crumbling like a house of cards. Lords who had once dictated terms were now competing for workers like modern tech companies compete for engineers. They offered higher wages, better working conditions, even plots of land. Some peasants became entrepreneurs, hiring out teams of workers to multiple lords. The concept of ‘human capital’ was being born, even if Adam Smith wouldn't name it for another four centuries.
By 1400, the transformation was complete. Europe had accidentally invented labor mobility, competitive wages, and something resembling a modern job market. All it took was the worst pandemic in human history.
One offshoot of this was the need for innovation. Less labor meant lords needed to become entrepreneurial and invest in new technology, and in new ideas. The mechanism of farming means better tools—axes, hoes, scythes, plows—the expansion of watermills and windmills, and growth in the use of spinning wheels, looms, and more. Between 1350 and 1400, investment in mechanical innovation increased by an estimated 300%.
If Austrian economist Joseph Schumpeter coined the term ‘creative destruction’ to explain how competition between individual firms drives growth in the broader economy, this fits more under ‘catastrophic innovation.’ A forcing function that moves innovation forward at the edge of the Grim Reaper’s scythe, where survival depends on getting better, fast. Another element was the fluid transfer of ideas. Knowledge that had previously been hoarded within guilds and regions was suddenly flowing across Europe like never before. Windmill technology that had taken 200 years to creep across the continent was now spreading in decades. New architectural techniques, improved farming methods, better manufacturing processes; all traveling at the speed of a determined craftsman with a head full of valuable skills and the freedom to use them.
Cities exploded. So did consumption. All of a sudden, for the first time in European history, ordinary people had disposable income, and they wanted to use it. This explosion in proto-consumerism led to the rise of markets, which became merchant hubs and, finally, bustling towns and commercial centers. This whole transition happened in the blink of an eye; London, for example, doubled its population during the years between 1350 and 1400.
But this transformation didn't happen without a fight. The old guard—nobles, church leaders, traditional power brokers—watched in horror as their world crumbled around them. They tried everything to stop it. Laws, threats, even divine authority. What they discovered would echo through history: you can't legislate away economic reality.
Imagine the conversations in the European power centers during this time. "Sire, the peasants are demanding triple wages. They're abandoning their fields. They're wearing... silk." The fightback was always going to come. Peasants and serfs winning the war on quality of life wasn’t just an economic occurrence; it was a total affront to the natural order of the universe.
Once the new reality began to set in, those in power tried their darnedest to deny, blame, and legislate their way out of this. England's Statute of Laborers, for example, passed in 1351, was a masterpiece of wishful thinking disguised as law.
It froze wages at pre-plague levels, banned workers from leaving their home parishes without permission, and made ‘excessive’ wage demands a criminal offense. | Actual graffiti from 1350 (not). |
This is the law of supply and demand at play. In some counties, officials gave up trying to arrest workers because there literally weren't enough jails. They were swimming upstream. The next stage of the resistance came from Europe’s great propaganda machine, the church. ‘God had placed everyone in their proper station, and attempting to rise above it was defying divine will,’ would be echoed in sermons across the land. But that again failed.
Smart lords quickly realized that resistance was futile and started competing for workers instead. They began offering employment contracts rather than feudal obligations, providing better housing, even giving workers shares in profits. These early adopters thrived in the new economy.
The culture, in general, was shifting faster than the establishment could control. Geoffrey Chaucer's Canterbury Tales, written in the 1380s, featured characters from all social classes traveling and storytelling together as equals—a radical concept that would have been scandalous 50 years earlier. These small changes all added to the cultural narrative. A narrative that was changing from ‘know your place’ to ‘know your worth.’
The Peasants' Revolt of 1381 in England was the moment when simmering tensions exploded. When rebel leader Wat Tyler met King Richard II and declared that "All men should be equal," he was articulating something unthinkable in the old world. | Lords in 1381. |
By 1400, the resistance had largely collapsed. The old world was gone forever. The invisible hand had won, as it always does. Smart lords had become employers, and successful peasants had become the middle class. Putting a close on a period of history that provided a blueprint for the prosperity that we're still enjoying today.
Here's the thing about history: it doesn't repeat, but it definitely does rhyme. And right now, in 2025, we're hearing some very familiar verses. The Black Death taught us something profound about how economies evolve: worker power isn't a zero-sum game. When ordinary people have leverage, everyone benefits. It's a lesson we keep forgetting and then dramatically rediscovering, usually during moments of crisis that force us to question everything we thought we knew about how work should work.
Look at any period of broad-based growth in human history, and you'll find the same formula hiding underneath: Crisis turns into worker leverage, breeds innovation, leads to prosperity.
The post-WW2 labor shortages led to union power, the cementing of the 40-hour work week (it was technically already law), and employee healthcare. This boom helped to grow the American middle class to what it is today. Fast forward to the 1960s: the tech revolution gave rise to the innovative hub known as Silicon Valley.
The tech industry's obsession with stock options is another example. Employee equity barely existed before the 1950s, when a few semiconductor companies started offering shares to attract scarce engineering talent. By the 1970s, startups were using stock options as currency because they couldn't afford to match IBM's salaries.
This wasn’t due to a sudden altruistic turn from leaders in business and tech. It happened because of competitive forces. Those same forces the lords of the 1300s felt. A reminder that the people have the power; they always have, and they always will.
The Black Death is still alive today: The CDC claims there are, on average, seven cases of Yersinia pestis (Black Death) per year in the U.S., with only a fraction fatal.The original 3-day work week: Well, not exactly. A serf would usually work 2-3 days of unpaid labor working the lord’s land, and 2-3 days tending to their own little sliver of turd. A typical day might look something like what you see below.
| | |---|---| 4–5am | Wake up with the rooster. Basic wash, no real breakfast. | 5–11am | Work the lord’s fields or tasks (dig, sow, reap, repeat). | 11–12pm | Break: small meal, ale, rest under a tree. | 12–3pm | More labor (either lord’s or own). | 3–5pm | Finish up, fix tools, maybe tend to animals. | Evening | Basic meal, religious observance, collapse into straw bed. |
The pestilence traveled fast: Actual land speed is estimated to have been between 1 and 8 miles per day depending on terrain.Rats may have been framed: The plague’s bacterium was blamed on rat fleas, but recent research suggests human lice and fleas could have been the main carriers.
‘Ring Around the Rosie’ might be plague-coded: Scholars debate it, but the rhyme: ‘Ring around the rosie, pocket full of posies, Ashes, ashes, we all fall down’ is possibly referring to the plague, masking the smell, cremation and death.Flagellants (word of the day) ran wild: The plague caused religious fanatics known as flagellants to march town to town whipping themselves to atone for humanity’s sins.
Effects of the Black Death on Europe- April, 2020How the Black Death made the rich richer- July, 2020
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