A U.S. Border Patrol agent inspects the bank of the Rio Grande River across from Mexico on Sept. 8, 2014, near McAllen, Tex.(Photo by John Moore/Getty Images)

First it was illegal immigrants coming to take our jobs. Then criminals. Then the threat of thousands of undocumented children.

After that cameclaims from conservative politicians and pundits that Islamic State militants would mount attacks on the United States from its southern boundary.

Now the outbreak of Ebola in West Africa is the latest justification for closing down the U.S.-Mexico border. Of nearly 4,000 documented cases of Ebola worldwide, only one has been diagnosed in the United States: that of Thomas Eric Duncan, the Liberian man who died earlier this week at a Dallas hospital.

Duncan arrived on a plane from Liberia. But that hasn’t stopped people from stoking fear that Ebola will spread to the United States via our border with Mexico, a country that has seen exactly zero cases of Ebola thus far. Few seem as concerned about Ebola entering the United States via Canada, our less politically-fraught border to the north.

Soldiers from the U.S. have arrived in the Liberian capital Monrovia to help fight the spread of the deadly Ebola virus. They are building a 25-bed Ebola isolation center for health-care workers infected with the disease. (Reuters)

Republican Senate candidate Scott Brown (R) said in a press release Thursday that Ebola “underscores the need to secure our borders,” specifically with Mexico. He warned that “we have already seen people with Ebola arrive in the United States through normal channels and the type of havoc that can create.”

In a debate with Sen. Kay Hagan (D-N.C.) on Tuesday, Republican candidate Thom Tillis said: “Sen. Hagan has failed the people of North Carolina and the nation by not securing our border … Ladies and gentlemen, we have an Ebola outbreak, we have bad actors who can come across the border. We need to seal the border and secure it.”

On his Fox TV show last week, former Arkansas Governor and one-time presidential hopeful Mike Huckabee said: “We’ve seen our borders routinely ignored. So if someone with Ebola really wants to come to the U.S., just get to Mexico and walk right in.”

Marine Corps Gen. John F. Kelly, the military’s top official responsible for Latin America, warned on  Monday “there’s no way we can keep Ebola [contained] in West Africa.” If the disease spreads to Latin America, “it’s literally, ‘Katie bar the door,’ and there will be mass migration into the United States,” Kelly said. He also warned smugglers would traffic infected people into the United States.

The drumbeat of an Ebola border crisis has been building for several months.

In July, Georgia Rep. Phil Gingrey (R) sent a letter to the Centers for Disease Control and Prevention (CDC) expressing concerns about “reports of illegal migrants carrying deadly diseases such as swine flu, dengue fever, Ebola virus, and tuberculosis.” Later, when questioned about the letter by NBC’s Luke Russert, he admitted: “I can’t tell you specifically that there were any cases of Ebola, I don’t think there were.”

In August, Rep. Todd Rokita (R-Ind.) told WIRC radio he and Rep. Larry Buschon (R-Ind.) were worried South American migrants might bring Ebola to the United States, according to the National Journal.

“President Obola” wants us to suffer for the sin of slavery

A more radical idea making the rounds on right-wing talk radio is the notion that liberals and President Obama think Americans deserve to suffer from an Ebola outbreak as punishment for our slaveholding past. (Liberia was established in the 19th century as a country for former slaves.)

Rush Limbaugh told his listeners: “The people in Liberia only went there because they had to get out of here ’cause they were slaves … Therefore if Ebola ends up here, it’s only payback, folks … Unfortunately we have elected people in positions of leadership who think this way. The president is one of them.”

In an interview with conservative news Web site World Net Daily, Phyllis Schaffly said of Obama: “If Africa is suffering from Ebola, we ought to join the group and be suffering from it, too. That’s his attitude.”

Another conservative radio host, Michael Savage, gave the president a new nickname: “President Obola.” “There’s not a sane reason to take 3,000 to 4,000 troops and send them into a hot Ebola zone without expecting at least one of them to come back with Ebola — unless you want to infect the nation with Ebola,” Savage said.

“It rises to levels of treason,” he added. “It actually exceeds any level of treason I’ve ever seen.”

The idea that Democrats want Ebola to spread to the United States isn’t just being spread by talk-radio hosts. “We used to have quarantines of serious diseases that would kill people, but this day and time, gee, we don’t want anybody to feel like they’re being left out,” Rep. Louie Gohmert (R-Tex.) told Newsmax on Thursday. “Some of my Democratic friends, including this president, feel like we want everyone to feel included. So let’s don’t quarantine, let’s don’t close our borders.”

A familiar pattern

The buildup of fear around Ebola has followed a similar pattern to alleged threats from Islamic State militants. That too has been a talking point on the Republican campaign trail.

Arkansas Republican Rep. Tom Cotton, who is running for Senate, said at a town-hall meeting the Islamic State is actively working with Mexican drug cartels to infiltrate the United States. ““They could infiltrate our defenseless border and attack us right here in places like Arkansas,” he said.

A campaign ad for Georgia Senate candidate David Purdue (R) claims “terrorism experts say our border breakdown could provide an entry for groups like ISIS,” an acronym for the Islamic State.

Texas Republican Rep. Ted Poe claimed in August the Islamic State and the cartels were “talking to each other.”

Rep. Duncan Hunter (R-Calif.) said on Fox News this week “at least ten ISIS fighters have been caught coming across the Mexican border in Texas.”

On Wednesday, the Department of Homeland Security (DHS) released a statement calling claims anyone with ties to the Islamic State had been apprehended at the Mexican border “categorically false” and “not supported by any credible intelligence or facts on the ground.” A U.S government official told CNN the detainees Rep. Hunter were referring to are actually members of the Kurdish Workers Party, a group fighting against the Islamic State in Syria.

Secretary of Homeland Security Jeh Johnson addressed rumors on Thursday of suspected terrorists trying to cross into the United States. Johnson said four individuals, claiming to be members of the Kurdish Worker's Party, were arrested for unlawful entry. (Reuters)

Previously, DHS said there is “no credible intelligence to suggest that there is an active plot by [the Islamic State] to attempt to cross the southern border.”

Government officials have similarly downplayed the threat of Ebola spreading via the southern border. “The bottom line is we know how to stop it and it’s not going to spread widely in the U.S.,” CDC Director Thomas Frieden said Sunday, according to MSNBC. He also said sealing the border is useless. “I wish we could get to zero risk by sealing off the border. But we can’t. The only way we are going to get to zero risk in this country is by controlling it in Africa,” he said on Fox News last week.

Soldiers from the U.S. have arrived in the Liberian capital Monrovia to help fight the spread of the deadly Ebola virus. They are building a 25-bed Ebola isolation center for health-care workers infected with the disease. (Reuters)