Professor Dave again, let’s get into some plagues.
The Plague, also known as the Black Death, is beyond infamous.
It is one of the deadliest, most devastating diseases mankind has ever seen.
Not only has the Plague killed an incredible number of people since it first broke out,
but it has also had a significant impact on art, economics, and pop culture over the centuries.
It was even recorded in the Old Testament, so it is literally biblical.
Let’s learn more about it now.
The Plague is an acute infectious disease with high mortality rate, meaning that it
kills many of the people it infects.
It has caused three major world pandemics.
Each time, this disease killed incredibly large numbers of people, and according to
some historians, irreversibly changed the fabric of society.
The first major pandemic, The Justinian Plague in the year 541 AD, spread from central Africa
to Egypt and the Mediterranean, killing an estimated 5,000 to 10,000 people per day at
its peak, and nearly 50 million people overall, or almost a quarter of everyone alive.
Next, the Black Death killed one-third of the population of Europe in the mid-1300s.
Imagine the horror of 1347, when 12 ships from the Black Sea arrived in Italy, with
everyone onboard unexpectedly either dead or very close to dead.
Things got worse from there, and five years later, the Plague had spread, killing over
25 million people across Europe, and another 25 million in Asia and Africa.
The third pandemic began in the late 1800s and didn’t end until 1959, killing over
15 million people across China, Hong Kong, India, Australia, and the Americas.
So we are talking about huge percentages of the global population being wiped out by this
disease, making it no surprise that we refer to it as THE Plague.
You might think that this horrific disease must be extinct by now, but the Plague is
still alive and well, causing outbreaks across the world even today.
The difference is that we know so much more about it now, and it can be cured if treated quickly.
So what does it look like?
The most common forms of Plague are: Bubonic Plague, Septicemic Plague, and Pneumonic Plague,
all of which are very serious.
In the case of Bubonic Plague, patients experience very sudden onset of fever, chills, headache,
and one or more painfully swollen lymph nodes, called “buboes.”
These relatively large buboes form in the neck, arms, or very commonly in the groin,
with potential to spread to the rest of the body.
With Septicemic Plague, the bacteria have moved into the blood.
Patients with septicemic plague experience bleeding from their mouth, nose, or bottom,
bleeding under the skin, blackened and/or dying skin, and vomiting, diarrhea, stomach
pain, and shock.
Finally, Pneumonic Plague is the rarest form, in which the bacteria spread to the lungs.
A patient might experience shortness of breath, pneumonia, bleeding, or bloody cough.
Of the three types, pneumonic plague is the most serious, and is the only type that can
spread in the air from person-to-person.
So what in the world causes it, exactly?
The causative agent of Plague, Yersinia pestis, wasn’t discovered until the 1894 pandemic
by French bacteriologist Alexandre Yersin.
Now we know that this bacterium is gram-negative, it can grow in the presence or absence of
oxygen, and it’s a member of the Enterobacteriaceae family.
A common characteristic of Yersinia species is their ability to survive in phagocytes,
which are a type of immune cell that typically engulf and destroy bacteria.
Instead of getting sucked into a phagocyte and killed, these bacteria secrete proteins
that are able to disarm the phagocyte.
They essentially take over the immune system from here, triggering cell death called apoptosis
in immune cells called macrophages, and suppressing other immune processes that would otherwise
signal distress to the body, such as cytokine production.
Many species of Yersinia carry plasmids encoding additional virulence genes as well, which
allow them to double down on their deadly cell takeover.
All Yersinia infections are zoonotic, meaning that they’re passed from an animal or insect
to a human.
Humans are simple accidental hosts.
In the case of Yersinia pestis, there are two forms of infection.
Rats are the natural reservoir, meaning the habitat where the bacteria normally live,
for urban plague.
Instead, squirrels, field rats, domestic cats, and rabbits are the natural reservoir for
sylvatic plague.
The way it works is this: the bacteria maintain their existence in a cycle involving rodents
and fleas.
Fleas become infected during a blood meal from an infected rat.
The bacteria then replicate in the gut of the flea, and the flea can transmit the disease
to another creature when it goes in for another blood meal.
In general, the plague bacteria can be transmitted to humans through bites from an infected flea,
contact with contaminated tissue or fluid such as a hunter skinning an infected animal
without protective gear, or infectious droplets from an infected person’s coughs.
Transmission by infectious droplet hasn’t been documented in the U.S. since 1924, but
happens fairly commonly in developing countries.
Once infected, it typically takes 7 days or less after a flea bite for a person to start
showing symptoms of bubonic plague.
In the cause of pneumonic plague, the incubation period is typically just two or three days.
These patients are highly infectious and face a mortality rate of above 90% if left untreated.
The good news is that although the plague is still serious, it’s absolutely treatable
with antibiotics.
The earlier a patient seeks medical care, the better their chances for a full recovery.
The Plague is a prime example of how important science is to our civilization.
The ominous and mysterious force that nearly decimated our entire species only a few hundred
years ago is now just another well-documented and understood microorganism.
We know what it is and what it does, and we know what to do about it, thanks to our understanding
of the microbial and molecular worlds.
Whatever dangers lie ahead for mankind, science will continue to be our first line of defense
in the millenia to come.