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This story was updated July 31 at 1:57 p.m. EST

The U.S. on Thursday issued travel warnings for three countries hit by the Ebola outbreak: Guinea, Liberia and Sierra Leone.

The warning to avoid non-essential travel from the Centers for Disease Control and Prevention (CDC) was the first the agency has issued since 2003, which was in response to SARS in Asia.

Ebola has killed at least 729 people since March of this year, making this the worst ever outbreak of the disease. Its origins are traced to forests in eastern Guinea, beginning in February.

The CDC is alerting healthcare workers in the U.S. to a potential threat of Ebola, however the organization believes it is unlikely it would spread in the U.S. White House press secretary Josh Earnest said Thursday the government may facilitate bringing two American aid workers with Ebola back to the U.S. to ensure their treatment.

The virus causes a hemorrhagic fever with a fatality rate up to 90%. The outbreak is currently at 60% with 1,323 people affected, according to the World Health Organization (WHO). Ebola is spread through direct contact with blood, bodily fluids and tissues of infected animals and people.

The disease has currently only affected western Africa, where regional airlines have cancelled flights to affected towns and health officials have announced potential precautions like doing temperature scans on travelers.

The WHO announced Thursday a $100-million response plan to this unprecedented outbreak of Ebola. WHO Director-General Margaret Chan will meet with government officials of affected countries in Conakry, Guinea on Friday.

Air travel and infectious disease

In 14th century Europe, the Black Plague spread moved across the continent as people spread the virus as they came in contact with others. Transportation at the time, according to medieval scholars, was slow, based on the speed of one’s horse and on the conditions of deteriorating roads.

Travel was a decidedly slow process, and communicative diseases moved across cities and countries at the same pace. The spread of the Black Death happened over the course of a few years, and in an easily understood geographic way:

When travel is by land, the speed at which a disease will reach more locations can be measured by distance. If travel happens at about 4 mph, whether by foot or on horse, disease spread is easily determined by distance.

Fast forward to 2014, and the spread of disease is better understood based on the speed of air travel, rather than geography. Major airport hubs are connected faster than less connected cities and towns, because it takes less time to get them if you are starting in a random place on the planet. Our remote locations are usually more connected to major transportation hubs — London, Dubai, New York — than they are to other remote locations that may be geographically closer.

That makes the modern spread of an infectious disease look like this:

The potential speed at which a highly infectious disease could spread around the world is terrifying enough to inspire horror movies, but a worst-case scenario is more fit for film than real life. The threat of a quickly-spreading airborne disease is taken very seriously by governments and health agencies including the WHO and CDC.

As part of the emergency response, WHO will initiate house-to-house searches for victims. In Liberia, officials have closed schools and are considering quarantines of different communities.

The WHO is working to make sure the disease does not spread beyond where it is currently, and it is tracking anyone who came into contact with victims on airplanes or buses.

Airlines take their cues from WHO and CDC officials.

If an airline crew member sees a passenger boarding that is obviously ill, they tell the captain, who then decides if the passenger “is fit to travel, needs medical attention or presents a danger to other passengers and crew or to the safety of the aircraft.”

“The safety and security of our customers and crew is always our top priority,” a British Airways spokesperson told Mashable. “We fully comply with the guidance provided by local health authorities and continue to monitor the situation closely.”

Read more: http://mashable.com/2014/07/31/us-travel-warning-ebola-africa/