For most of human history,
medical workers sought to treat diseases or cure them.
The rise of vaccination in the 19th century
enhanced the potential to prevent people
from contracting illnesses in the first place.
But only in recent decades did it become possible
to ensure that a particular disease never threatens humanity again.
The story of smallpox,
the first and, so far, the only disease
to be permanently eradicated from the world,
shows how disease eradication can happen and why it is so difficult to achieve.
Smallpox emerged in human populations thousands of years ago
as a contagious virus that spread rapidly,
primarily through close, face to face contact,
causing fever, aches and rashes.
It killed up to 30% of its victims
and often left survivors with life-long disfiguring scars.
The devastating impact of smallpox was so great
that several cultures had religious deities specifically dedicated to it.
In the 20th century alone,
it is estimated to have killed more than 300 million people worldwide.
With the effective deployment of vaccination,
the number of cases began to decrease.
By seeking out infected individuals,
isolating them,
and vaccinating their contacts to prevent further transmission,
scientists realized that the spread of the disease could be haulted.
In fact, because smallpox could only survive in human hosts,
vaccinating all of an infected persons' potential contacts
would stop the virus dead in its tracks
and eliminate it from that region.
Once this strategy had succeeded
in ridding most industrialized countries from disease,
health officials realized that eradicating it worldwide was within reach.
But this was not an easy process,
proving especially difficult in places suffering from poor infrastructure
or civil wars.
The eradication effort took decades
and involved millions of people working together,
from world leaders and international organizations
to rural doctors and community workers.
In India, one of the last strongholds of the disease,
health workers visited every one of the country's 100 million households
to search for cases.
Through this unprecedented worldwide effort,
in which even rival superpowers cooperated,
smallpox was finally declared eradicated in 1980,
saving approximately 40 million lives over the following two decades.
There were several factors
that made smallpox an ideal candidate for eradication.
First, humans are essential to the smallpox lifecycle,
so breaking the chain of human to human transmission
causes the virus to die out.
In contrast, many other pathogens, like ebola or the bubonic plague,
can survive in animal carriers,
while the bacteria that cause tetanus can even live in the soil.
Secondly, individuals infected with smallpox
displayed a characteristic rash, making them easy to identify,
even without a lab test.
The lack of such practical diagnostic tools
for diseases with non-specific symptoms, or that have long incubation periods,
such as AIDS, makes their eradication more difficult.
Third, the availability of a smallpox vaccine
that provided immunity for five to ten years in a single dose
meant that there was an effective intervention
to stop the virus from spreading.
And finally, the initial success of several countries
in eliminating the disease within their borders
served as a proof of principle for its eradication worldwide.
Today, the same criteria are applied to determine
whether other diseases can be similarly eliminated.
And even though smallpox remains the only success story thus far,
several other pathogens may be next in line.
Great progress has been made towards eradicating guinea worm disease
simply by use of water filters.
And vaccination for polio,
which previously disabled hundreds of thousands of people each year
is estimated to have prevented 13 million cases of paralysis,
and 650,000 deaths since 1988.
With a 99% drop in infections since the eradication effort began,
one final push is all that is needed
to ensure that polio will never paralyze another child.
Disease eradication is one public health effort that benefits all of humanity
and challenges us to work together as a global community.
Beyond eliminating specific diseases,
eradication programs benefit local populations
by improving health infrastructure.
For example, Nigeria recently used facilities and personnel
from their polio eradication program to effectively control an ebola outbreak.
Further more, globalization and international travel
means that even a single infection anywhere in the world
can potentially spread to other regions.
By helping to protect others, we help to protect ourselves.
Disease eradication is the ultimate gift we can give to everyone alive today,
as well as all future generations of humanity.