When the US puts a major portion of a continent like Africa on its travel warning list, you have to wonder what data is behind it.

"[Travel warnings] are simply a reflection, a very factual and very objective reflection, of the security situation as we and our experts see it," according to Michelle Bernier-Toth, managing director for the US State Department's overseas citizens services.

She says decisions about whether to add a country to the warning list are made through a "collaborative process" that is based on information collected from embassies and consulates overseas, local governments, the intelligence and security community, media reports, and from "wherever we can get it."

And the governments of countries on the list typically hate it and complain. For one thing, travel warnings do not have an expiration date. Bernier-Toth says they are only lifted when the US State Department determines that "the security situation or the conditions that led to the warning no longer exist."

These "Travel Warnings," which can be found at travel.state.gov, are the strongest mechanism used by the State Department to discourage travel to a country short of imposing an all-out ban. The vast majority of these warnings fall in Africa and have caused some nations to decry the long-standing US alert system.

Right now, according to the travel warnings issued by the State Department, US citizens should “strongly consider not going” to more than one third of Africa. This map shows the countries on the list in recent years and the rise and fall of American travel to the continent.

Credit: David Conrad

Kenya rebuked the United States and Britain earlier this summer for issuing warnings about travel to the east African country. After the alerts, tour operators cancelled all flights to Kenya's popular coastal town of Mombasa until October and sent hundreds of holidaymakers packing as a precaution.

The warnings and departures by tourists have caused considerable damage to Kenya's tourism sector, one that President Uhuru Kenyatta reportedly said is "on its knees" following the series of deadly attacks by the Islamic militant group al-Shabab, based in Somalia.

Bernier-Toth says countries shouldn't blame the US State Department warnings for declining tourism. "It's not the travel warning in and of itself that has an impact," she says, "but the situation on the ground."

Nevertheless, Kenya called the alert "unfriendly", saying it would increase panic and play into the hands of al-Shabab, which claims it is trying to cripple Kenya’s economy. The recent travel warnings are a vexing pattern for Kenya, which has found itself on the list every June for more than 10 years.

Similarly, while al-Shabab hasn’t launched a single attack in Burundi, the mere fact that the group has threatened to launch an attack there landed the tiny Central African nation on the list, along with more dangerous nations, such as Somalia and the Central African Republic.

Here is a map that shows where the US says Americans are at greatest risk in 2014, and another that shows where Americans have actually been killed by terrorism over the last 10 years.

One spoiler alert: terrorist attacks have killed as many US citizens in Texas as they have in all of Africa. Does that affect your travel plans?

Credit: David Conrad

*In a previous version of this story, the infographic titled "Rise and Fall of Travel to Africa by US Citizens" did not use a consistent scale across the two rows. This has been corrected. Thank you to Thom Stevenson for pointing this out!