The Plague Will Probably Return

Scientific concepts, discoveries, and nature/biological phenomena

What causes plague (and what “types” exist)

• Bubonic plague is caused by the bacterium Yersinia pestis (Y. pestis) and involves infection of lymph nodes, producing characteristic buboes.
• Plague can present in multiple forms depending on where the bacteria infect:
  • Septicemic plague: infects the blood; can lead to gangrene and tissue blackening.
  • Pneumonic plague: infects the lungs.
• Bubonic plague is described as the most survivable of the three forms.

Key scientific finding: the bacterium identified (1894)

Scientists discovered the cause of bubonic plague in 1894 as Yersinia pestis—not supernatural/astral explanations.

Ancient DNA (a major methodology used)

Multiple studies infer past plague spread by:

• Extracting and sequencing ancient Y. pestis DNA from human/animal remains (e.g., teeth, bones).
• Comparing mutations and genomic lineages across time and geography using statistics to estimate likely origins and movement pathways.

Oldest known cases and early spread (ancient lineages)

• 2021 study: evidence for the oldest known plague case from a man in present-day Latvia (>5000 years ago).
  • Ancient DNA suggested the strain was not super contagious.
  • Likely rodent-bite infection leading to septicemic plague and death within days.
• 2023 study: tracked plague in Britain ~4000 years ago using Y. pestis DNA from people buried at two sites.
  • Identified the Late Neolithic Bronze Age lineage (LNBA).
  • LNBA appears to have spread across Eurasia (Mongolia → Western Europe) roughly 2900 to 500 BCE.

How plague spread: flea transmission vs. animal-to-human spillover

A major adaptation for flea-based spread is described:

• A gene called ymt helps Y. pestis survive in a flea’s digestive tract.
• Evidence suggests flea-transmission capability became prominent more recently than the earliest LNBA spread.

2025 study (sheep/steppes):

• LNBA-era plague DNA found in a domesticated sheep from the Eurasian steppes.
• The plague strain matched human samples from the same region/time.
• Interpretation: the sheep likely contracted plague from a wild animal and then passed it to humans through handling and consumption.

Reported timeline pattern:

• The genetic mutation required for flea transmission was not found in samples older than ~3700 years, but is common in samples <3000 years.

The first “proper” plague pandemic: Plague of Justinian (starting 541 CE)

Plague of Justinian is described as:

• Often considered the first plague pandemic and the first recorded pandemic.
• Likely originating in Egypt and the Mediterranean.
• Death toll estimated at ~25–100 million.

Evidence updates:

• Contemporary symptoms led scientists to suspect Y. pestis.
• DNA confirmation strengthened in 2025 by Y. pestis DNA from eight teeth from a mass grave in Jordan.

Course:

• Initial outbreaks lasted to about 549 CE.
• The strain persisted for two more centuries, triggering ~18 additional waves, then disappearing.

Suggested general pattern:

• Plague strains often die out after a few centuries, potentially due to increasing host genetic resistance.

Black Death (1346 onward) and correcting origin myths

Earlier stories claiming the Black Death started in China and spread via the Silk Road are described as historically inaccurate. The narrative is linked to:

• A 14th-century poet writing a fictional trickster story that was later mistaken as history.

Modern ancient-DNA/genomic approaches (2022 study):

• Sequenced plague genomes from 14th-century victims and compared them to other regional genomes.
• Conclusion: the strain likely originated in Central Asia (not China).

Impact:

• Black Death lasted only a few years but caused at least ~one-third of Europe’s population to die.

Human genetic selection after the Black Death

Survivors passed on protective genes, but some variants may also increase autoimmune risk, including:

• Crohn’s disease

The script frames this as a trade-off:

• plague protection ↔ higher autoimmune disorder prevalence

Reintroduction of plague into Europe and reservoirs

• The strain re-entered Europe multiple times over centuries.
• A “reservoir” is described as an environmental niche where a pathogen persists in the wild.
• Possible reservoirs:
  • Likely rodents and other animals involved in transmission via fleas and animal movement.

Lineage evolution and genetic “less deadly after ~100 years” trend

After the pandemics, one lineage is described as:

• Continuing into modern strains (the ancestor of modern plague)

Another lineage is reported to have:

• Died out by the 1800s (reasons uncertain)

Proposed explanation (2025 paper) for changing severity:

• Mortality decreases are associated with fewer copies of the gene pla.
• pla helps the bacteria evade immune detection until reaching lymph nodes.
• Evolutionary idea presented:
  • More pla → faster/more deadly infection (less time to spread)
  • Fewer pla → longer host survival and less severe disease (more transmission opportunity)

Third pandemic: Late 19th century “Yunnan” outbreak

• A third plague pandemic began in Yunnan, China in the late 19th century.
• It spread across Asia and globally until the 1950s.
• Estimated deaths: ~12 million in about a century.

Plague today (not currently a global pandemic, but still present)

Hotspots cited:

• Madagascar
• Democratic Republic of the Congo
• Peru

Reported incidence is low:

• Typically <50 cases per year in those regions.
• United States: about ~7 diagnoses per year, with extremely low deaths.

Notable recent case:

• July 2025: a death in Arizona from pneumonic plague.

Additional notes:

• US reservoirs described as mainly rodents, especially prairie dogs, squirrels, and chipmunks.
• Advice included: do not touch wild animals.

Prevention and treatment (tools and limitations)

• Transmission prevention:
  • Masks are said to reduce transmission (especially human-to-human, relevant for pneumonic plague).
• Treatment:
  • Antibiotics work if given early, but some strains may show antibiotic resistance.
• Vaccines:
  • Few/limited vaccine options currently.
  • Main challenge: making a vaccine that is effective and affordable for mass manufacturing.
• Ongoing environmental risk:
  • A “fourth pandemic” is framed as plausible due to persistent reservoirs.

Additional environmental/biological persistence reservoirs

The script claims Y. pestis can persist in:

• Human body lice
• Soil
• Amoebas in water

Pets/animals:

• Cats are noted as susceptible to severe cases.

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Researchers or sources featured (explicitly named)

No individual researchers’ names are provided in the subtitles. Only a few entities are explicitly referenced:

• A Byzantine emperor (name not given) associated with Plague of Justinian (named after him; “the Byzantine emperor at the time”).
• Angela (credited as “the fact checker” for the script; not a plague researcher).
• SciShow (implied by “SciShow video,” but not a specific researcher).