a mysterious epidemic suddenly struck two central African towns,

killing the majority of its victims.

Medical researchers suspected

the deadly Marburg virus to be the culprit.

But what they saw in microscope images was an entirely new pathogen,

which would be named after the nearby Ebola river.

Like yellow fever or dengue,

the disease caused by the Ebola virus is a severe type of hemorrhagic fever.

It begins by attacking the immune system's cells

and neutralizing its responses,

allowing the virus to proliferate.

Starting anywhere from two to twenty days after contraction,

initial symptoms like high temperature,

aching,

and sore throat,

resemble those of a typical flu,

but quickly escalate to vomiting,

rashes,

and diarrhea.

And as the virus spreads,

it invades the lymph nodes and vital organs,

such as kidneys and liver, causing them to lose function.

But the virus itself is not what kills Ebola victims.

Instead, the mounting cell deaths trigger an immune system overload,

known as a cytokine storm,

an explosion of immune responses that damages blood vessels,

causing both internal and external bleeding.

The excessive fluid loss and resulting complications

can be fatal within six to sixteen days of the first symptoms,

though proper care and rehydration therapy

can significantly reduce mortality rates in patients.

Fortunately, while Ebola is highly virulent,

several factors limit its contagiousness.

Unlike viruses that proliferate through small, airborne particles,

Ebola only exists in bodily fluids,

such as saliva,

blood,

mucus,

vomit,

or feces.

in order to spread,

these must be transmitted from an infected person into another's body

through passageways such as the eyes, mouth, or nose.

And because the disease's severity

increases directly along with the viral load,

even an infected person is unlikely to be contagious

until they have begun to show symptoms.

While Ebola has been shown to survive on surfaces for several hours,

and transmission through sneezing or coughing is theoretically possible,

virtually all known cases of contraction have been through direct contact

with the severely ill,

with the greatest risk posed to medical workers

and friends or relatives of the victims.

This is why, despite its horrifying effects,

Ebola has been far less deadly overall than more common infections,

such as measles,

malaria,

or even influenza.

Once an outbreak has been contained,

the virus does not exist in the human population

until the next outbreak begins.

But while this is undoubtedly a good thing,

it also makes Ebola difficult to study.

Scientists believe fruit bats to be its natural carriers,

but just how it is transmitted to humans remains unknown.

Furthermore, many of the countries where Ebola outbreaks occur

suffer from poor infrastructure and sanitation,

which enables the disease to spread.

And the poverty of these regions,

combined with the relatively low amount of overall cases

means there is little economic incentive for drug companies to invest in research.

Though some experimental medicines have shown promise,

and governments are funding development of a vaccine,

as of 2014,

the only widespread and effective solutions to an Ebola outbreak remain

isolation,

sanitation,

and information.