You should be afraid of Ebola—if you live in some parts of West Africa. But here in the United States? Not so much.

With the first case of Ebola diagnosed in the country confirmed earlier this week, people are getting nervous. But Ebola is very unlikely to be a problem or cause a major outbreak here. One of the main reasons is that it is not as easily transmitted as other diseases. It does not travel through the air like influenza—to be infected you must come into contact with fluids from an infected person.

Even more importantly, until a person is showing signs of being sick—with symptoms like fever and nausea—they are NOT contagious.

Still, some are worried about the plane the infected man traveled on before he became symptomatic, and others are worried about coming into contact with his relatives in Dallas who've been in the same house as he has, but don't yet appear to be sick.

To help explain why these fears are unnecessary, we've taken a closer look at what the virus does in the human body, from transmission to infection to illness and death.


"The virus doesn't easily go from one person to the next. It seems like it does, maybe, because Ebola is scary. It is unknown and has a high fatality rate and requires isolation or quarantine and has no known cure," said biochemist Sharon Crary, of DePauw University. Crary studies the Ebola virus and worked with the CDC's Viral Special Pathogens Branch, where she was part of the response team for the 2000 Ebola outbreak in Gulu, Uganda.

Ebola isn't anywhere near as contagious as the flu, for example. Or measles, which is much more of a threat in the United States now that people are no longer routinely vaccinating their children. Scientists estimate that one person infected with measles can transmit the disease to as many as 18 others; for Ebola, that number is around two.

This is because unlike influenza or measles, Ebola isn't very stealthy. It can't spread through the air, and it isn't contagious before symptoms first show up, when a person might unknowingly be a walking disease distributor. Rather, the Ebola virus spreads through infected bodily fluids—such as blood, vomit, saliva, semen and feces—which need to come into direct contact with a mucous membrane (such as the inside of your eyelids, mouth, or nose) or a bit of broken skin.

This is why a major outbreak is unlikely in the United States. The hospitals here are equipped to handle a disease like this, and infection control officers are ready to slam the brakes on any potential spread. It should be relatively easy to contain.

Still, if you're a health care worker in West Africa, this is a serious concern, especially because tiny, unnoticed abrasions on the skin can be a portal for viral particles (hence the need for gloves and moon suits). In regions without the necessary supplies, education or infrastructure to halt the spread of the disease, outbreaks can be catastrophic. This is what's happening in the West African countries of Liberia, Guinea, and Sierra Leone, where more than 3,300 people have died since December.


Though it is not that easily transmitted between people, the Ebola virus (Zaire ebolavirus) is frighteningly deadly: The average fatality rate is around 50 percent, but some strains kill as many as 90 percent of the people infected. There's no specific treatment for Ebola, either. As the virus gradually claims control of a victim, it ignites a hemorrhagic fever that sometimes comes with horrific symptoms, including bloody diarrhea and vomit.

Ebola virus particles are long and skinny, and look like lethal microscopic noodles. When they get inside a person, the particles attack the immune system, liver, kidneys, and the cells that line blood vessels.

Once inside a cell, the virus begins to wage war. First, it makes many copies of its genome. Then, it hijacks the machinery that would normally help the host cell make its own proteins, and turns that into a viral protein production factory. These proteins then self-assemble into mature virus particles, which slip through the cell's membrane and head off in search of more cells to infect.

"This cycle continues so the number of infected cells in a human increases exponentially," Crary said.

The first symptoms of Ebola, such as headache, high fever, aches, and nausea, don't show up until enough cells have been infected with the virus. This can take a while.

And it isn't until these symptoms show up that a person becomes infectious. Scientists aren't quite sure why this is, but studies in primates have shown that there are no viral particles in blood plasma before the onset of symptoms (in monkeys infected with Ebola, this tends to happen around three days post-exposure). Early, pre-symptomatic viral loads were the highest in the spleen and lymph nodes—things that would be very, very tricky to come into contact with.

"It seems to take a lot of virus particles to exist inside a patient before that virus starts entering the bodily fluids to be accessible to another person," Crary said. "And this high virus [concentration] doesn't happen until later in infection, when symptoms are already starting to show."

This is the simple reason you can't catch Ebola by sharing an airplane, or a dinner table, or a house, with someone who isn't showing signs of the disease. However, it appears that some people are worried about riding in the same airplanes that carried the first U.S. Ebola patient from Liberia to Texas, despite the fact that the patient was not sick while he was traveling.

"Even though health officials maintain there is no risk to passengers, if you were a passenger on a plane that had carried an Ebola victim, it might be something you would want to know," claims a story from ABC News

In reality, the information is completely inconsequential.

"The man was not symptomatic," Crary said. "He didn't have enough virus in his body to be able to be shedding any virus from his body. So there is simply no way that there is any virus on that plane that he was on."

Sickness and Death

It wasn't until the patient had been in Texas for several days that he began showing symptoms—about nine days after doctors suspect he was infected. Normal incubation periods range from two days to three weeks, with the majority of patients showing signs of illness between seven and 10 days after exposure, says mathematical epidemiologist Gerardo Chowell-Puente of Arizona State University.

Though these early symptoms can mimic the onset of the flu, what's going on inside a patient's body is very different from what happens with an influenza virus. At this point, an Ebola patient's liver is being attacked, producing severe abdominal pain. Their blood vessels are gradually being broken down, which can lead to massive amounts of both internal and external bleeding. Organ failure might be setting in. As fluids leak from blood vessels and organs and accumulate in the body, blood pressure drops. Usually, it's a deadly combination of abyssal blood pressure, electrolyte imbalance, and organ failure that delivers the final blow.

Nobody knows for sure how long the Ebola virus can survive outside a host. But 2007 study suggests that viral particles can survive for at least six hours at sub-Saharan Africa room temperatures—only in fluids like blood. But what we do know is that the dead are potent viral incubators that remain infectious for days, and disease transmission during traditional funeral rites is one of the ways this West Africa outbreak has persisted.

In the current West Africa outbreak, each infected person transmits the disease to an average of 1.5 or two susceptible people, Chowell-Puente says. This is one of the reasons some experts have a grim forecast for that outbreak, which could top 270,000 cases by the end of the year without proper intervention.

It's easy to see how, in an age where airplanes make continent-hopping as simple as owning a passport and buying a ticket, people worldwide might be frightened about the spread of deadly diseases. And it's not hard to imagine a situation in which the Ebola virus could gain a foothold outside of Africa. But in the United States, there's no reason to be scared of Ebola.