Smallpox is considered to be one of the deadliest diseases the world has ever known.
No one knows the true origin of the disease, but artifacts dating back to the 3rd century
BCE show evidence that is believed to be indicative of smallpox.
The disease seems to have spread across the globe in tandem with the growth of civilizations,
due to expanding trade routes, and continuous exploration.
At least a billion people have died from this disease over the course of human history.
Ok, so what causes it?
The virus that causes smallpox, which is called the variola virus, falls into the poxvirus family.
Poxviruses are the largest of all viruses, so large that they are almost visible using
light microscopy.
They form complex structures to protect large double-stranded DNA viral genomes, and replicate
in a unique way compared to other DNA viruses.
Poxviruses replicate in the host cell cytoplasm, which means they must encode the enzymes required
for messenger RNA and DNA synthesis in their viral genome.
These viruses tend to also encode proteins for nucleotide scavenging or immune escape
mechanisms, which explains their large size.
Variola virus is spread from person to person through airborne droplets of saliva from an
infected person.
After being inhaled, the variola virus replicates in the upper respiratory tract without causing
any symptoms or contagion.
The virus then infects immune system cells called macrophages, which bring the virus
into the lymphatic system, which then carries the virus to lymph nodes.
The virus then replicates and initiates viremia, which spreads the virus through the bloodstream
into the liver, spleen, and other organs.
There are several clinical forms of smallpox, each with different characteristics.
Ordinary smallpox is also called “Variola Major.”
The incubation period for the variola virus in ordinary smallpox is somewhere between
7 to 19 days before high fever, headache, backache, severe abdominal pain, pharyngitis,
and/or malaise kicks in.
After 4 days, the fever typically subsides and rash lesions develop on the skin.
Lesions begin on the oropharynx, spreading to the face and extremities, then trunk, followed
by the palms and soles of the hands and feet.
These lesions progress from macules to papules to vesicles over the next 4-5 days, then forming
scabs, which shed about 14 days after the rash began.
Some cases also involve intense vomiting, diarrhea, and excessive bleeding.
Pockmarks and scarring, especially of the face, are the most common consequence from
smallpox, but rare instances of blindness, encephalitis, spontaneous abortion, or infertility
have also been reported.
The good news is that recovery from smallpox gives prolonged immunity to reinfection with
variola virus, and the virus does not persist in the body after recovery.
Just like polioviruses, variola viruses only infect humans.
Other members of the poxvirus family primarily infect cows, sheep, or goats, though accidental
or occupational exposure via zoonosis is possible.
Even better news is that smallpox is the only human disease to have been completely eradicated
by vaccination.
It all started in 1959, when the World Health Organization hatched a plan to eliminate smallpox.
Due to a shortage of funds and support, smallpox remained widespread and caused regular outbreaks
across South America, Asia, and Africa by 1966.
The Intensified Eradication Program began in 1967 and made steady progress, eliminating
the virus in North America and Europe.
The last case of naturally acquired smallpox infection was reported in 1977, and the eradication
of the disease was officially acknowledged in 1980.
Vaccines remain available to protect against smallpox, but routine vaccination for smallpox
ended in the U.S. in the early 1970s since the disease was no longer a threat.
This remains one of the greatest triumphs in the history of medicine, or even in the
history of humanity in general.