mass sickness and devastation.

But as much tragedy as the disease has caused,

it also helped drive crucial scientific and social progress.

Plague is an infectious disease

caused by the bacterium Yersinia pestis.

It mainly affects rodents, and spreads by way of insects.

Because of these insect carriers,

plague has been passed onto humans

with devastating consequences.

Three major plague pandemics have occurred in human history.

And while they occurred centuries apart,

they shared similar traits

that paved the way for the spread of disease.

One cause of plague pandemics

was the rise of international trade.

Trade routes connected once-isolated communities

and created large economic networks.

But by facilitating the movement

of goods between communities,

trade routes also facilitated the movement of germs.

International trade was an impetus

for the first plague pandemic on record,

the Plague of Justinian.

In the sixth century, outbreaks began in Egypt

and, thanks to land and sea trade routes,

they spread throughout the Byzantine Empire.

Named after the emperor at the time,

the Plague of Justinian is estimated to have wiped out

about half of Europe's population.

Growing economies also made way for urbanization

and a rising urban population.

This resulted in crowded neighborhoods

and the accumulation of waste,

which created unsanitary living conditions.

Cities and their residents essentially became incubators

for germs and diseases.

This was particularly evident in the second

and most infamous plague pandemic.

In the 14th century,

Europe was experiencing an economic and population boom,

especially in cities.

Proper waste management did not exist at the time,

making cities vulnerable to disease.

After trade routes brought plague from Asia,

where it killed millions in China and the Middle East,

the disease wiped out about a third of Europe's population,

earning itself the moniker the Black Death.

What also aided in the transmission of the disease

was the lack of medical knowledge.

For most of human history,

the cause of illnesses, germs, was unknown,

making sicknesses like the plague a mystery.

This lack of knowledge drove the spread of disease

as recently as the 19th century.

Outbreaks in northwest India

eventually reached major port cities in China.

In just over a century,

plague was exported throughout the globe

and caused outbreaks in every continent except Antarctica,

making it the most widespread pandemic in history.

This plague pandemic, however, was the last.

In 1894, scientists discovered the bacteria

behind the plague outbreaks.

Their discovery helped further developments in microbiology,

medicine, urban planning, and sanitation methods,

which led to the treatment and prevention of the disease.

Economic expansion, urbanization,

and a lack of medical knowledge

contributed to the disastrous spread of plague.

In turn, however, the disease helped catapult

crucial advancements in science and public health,

very well making plague pandemics a thing of the past.