The number of Ebola cases has been growing – and growing by a lot — each day since the World Health Organization declared a public health emergency on Saturday. The latest toll? More than 600 suspected cases and 139 suspected-Ebola deaths.

The vast majority of the cases are in a province in northeastern Democratic Republic of Congo — a remote place struggling after decades of bloody conflict. There are also two cases in Uganda’s capital. The World Health Organization has identified the strain of Ebola as a rare one and says the outbreak could have started months before it was detected.

“This is an example of a perfect storm,” says Dr. Abraar Karan, an infectious disease physician and faculty at Stanford University.

Since it’s been more than a decade since the large West Africa Ebola outbreak, here is what you need to know about this virus and what’s on the minds of infectious disease experts as they look at the current outbreak.

Where and how do Ebola outbreaks start?

Geographically, this is easy to answer: Ebola outbreaks have almost always started in either east and west Africa. By far, the Democratic Republic of the Congo has detected the most outbreaks. This is its 17th since 1976.

Exactly how humans pick up the virus remains a question mark.

“We don’t know for sure where it comes from but we have suspicions,” says Karan, whose team has been studying Ebola and related viruses in Kenya for several years.

The leading guess, he says, is that people get Ebola from eating bat meat or being exposed to bat guano – or excrement. This could happen when miners go into caves.

“A number of animals have also tested positive for antibodies, so certain types of deer called duiker that eat meat have been implicated. Non-human primates have shown antibodies,” he says.

What tends to happen is that one person gets it from an animal — which is called a spillover — and then that individual spreads it to other people.

What does the virus do to people? 

Dr. Nahid Bhadelia cared for more than 500 Ebola patients in the West Africa outbreak a decade ago.

“One of the biggest things I learned during that period of time is that there’s a whole range of presentations of Ebola. In some cases, it may present quite mildly, almost like a flu-like syndrome, and people get better,” says Bhadelia, who is an infectious diseases physician and director of Boston University Center on Emerging Infectious Diseases.

She says in the early phases of the infection Ebola often resembles other infectious diseases, such as malaria and typhoid. Symptoms can include nausea, diarrhea and fever.

What’s concerning is what happens when the disease progresses — but it’s not the way it’s depicted in movies, like the 1995 film Outbreak.

“In many Hollywood movies, you may see Ebola portrayed as bleeding out of the eyes. I have to tell you, after seeing hundreds of Ebola patients, I have yet to see that,” says Bhadelia.

Instead, she says, there are “massive amounts of diarrhea and vomiting” that’s often bloody. Many patients die from the body going into shock and organs shutting down, “driven by the immune system of the patient revving up in response to the virus.”

She points out that the likelihood of survival depends on how quickly the patient gets medical care as well as the quality of that care, which can include supportive care or even monoclonal antibodies. Monoclonal antibodies are artificially produced antibodies that mimic the body’s natural antibodies and help stop the virus.

“In West Africa, we had a mortality between 50 to 70%,” she says. By comparison, Americans infected there and brought back to the U.S. for care saw mortality rates of less than 20%. “This really shows the difference in terms of good medical care as well as targeted therapeutics.”

What’s known about this particular strain of Ebola?

Different strains of the Ebola virus have different mortality rates.

For the Zaire strain, responsible for the large 2014-2016 West Africa outbreak, up to 90% of cases are fatal if untreated, according to the U.S. Centers for Disease Control and Prevention. That’s the strain that took off in the West Africa outbreak but this eye-popping number is not the case for the Bundibugyo virus — the strain identified in the current outbreak.

“If there’s any upside to this, the data that we have on the Bundibugyo virus from previous outbreaks, the case fatality rate has been a little bit lower — not comfortingly low — but lower than some of the other strains or species of Ebola,” explains Dr. Daniel Bausch, visiting professor at the Geneva Graduate Institute.

From past outbreaks, this strain seems to have a fatality rate of between 30 and 50%, says Bhadelia. However, the challenge is that there are only two known past outbreaks of Bundibugyo, so there’s not a lot of data.

The other big challenge is that there are no vaccines or specific treatments for this particular strain of Ebola. This stands in contrast to the Zaire strain of the virus, where there are two vaccines licensed as well as monoclonal antibodies.

The lack of medical options has worried a lot of infectious disease experts — but some are holding out hope. “That, of course, is an impediment but we’ve controlled lots of Ebola outbreaks in the past without having a vaccine or a therapeutic,rehydration,” Bausch points out. It’s only in the past handful of years that these tools have been an option for the Zaire strain.

Without such options, medical professionals rely on other approaches to containing the virus and caring for the patients, including supportive care such as rehydration. Bausch says control measures include very good infection control and something called contact tracing — tracking down people who’ve interacted with someone who’s been infected.

How contagious is Ebola?

First, the good news. “It doesn’t transmit by the airborne route,” says Karan. “So, it’s not nearly as contagious as COVID-19 or measles.”

Bhadelia says the data back that up: “The number of people that one person [with Ebola] infects is about two as opposed to measles where the number is about 18,” she says. “Measles is a lot more transmissible, although the mortality rate of many of the Ebola species is much, much higher.”

Ebola typically spreads between people through contact with bodily fluids: saliva, blood, semen, diarrhea.

People with Ebola are not considered infectious until they start showing symptoms. “As the person gets sicker, there is more virus in their bodily fluids,” says Bhadelia. She says this highlights why good infection control in the hospital or for caretakers at home is key – that’s things like gloves, gowns and masks.

Death does not end the risk. “There’s a lot of virus in those bodily fluids in cases where patients pass away,” she says. “Unfortunately, that is when they have the most amount of virus in their bodies, which is why safe burials were so important.”

In the 2014-2016 West Africa outbreak, several calculations suggest that funerals and burial traditions were linked to well over 50% of the cases. For example, in Liberia and Sierra Leone, some mourners bathe in water that was used to wash corpses. Another tradition involves sleeping near the corpse for several nights, according to the World Health Organization.

If a person recovers from the virus, there is also the chance the virus can hide out in certain parts of the body that are protected from the immune system — like in semen. This has been known to reignite outbreaks in the past and is one reason survivors have to be monitored in the months and years following an infection.

Why have many past Ebola outbreaks “fizzled out”?

“So most Ebola outbreaks fizzle out,” says Stanford’s Karan — albeit not without a tragic cost in human life.

There are two reasons why. First, outbreaks usually occur in rural areas where there are fewer opportunities to spread the virus to others. The reason it seems to start in remote areas is because these communities tend to have more close interaction with wild animals. 

Second, the virus is highly fatal, so when people get sick they often die before they can spread it very far.

He starts worrying when the virus gets into big cities.. That’s what happened in the 2014–2016 outbreak in West Africa — the largest Ebola outbreak on record. There were more than 28,600 cases reported and 11,000 deaths, according to the World Health Organization.

And, Bausch adds, that with modern connectivity, road networks and centralized hospitals, you “can’t count on ‘burn out’ these days” even if the outbreak starts in a remote area.

What makes this outbreak worrisome?

Many infectious disease experts are watching the current outbreak unfold with growing alarm.

“My concern is very high,” says Bhadelia.

What’s fueling that feeling?

First, this strain does not have vaccines or specific treatments.

Second, this outbreak took a while to detect and has already crossed borders and spread to several big cities, including to Kampala, the capital of neighboring Uganda, and to the regional hub of Goma in the DRC.

“To discover so many patients in so many different cities and towns so far away from each other, it tells me this has been going on for a while,” says Bhadelia. “In many cases, when patients passed away their bodies were transported back to the homeland as would be expected culturally to be buried in their home.” She worries about how those bodies were handled and whether more people were exposed to the virus in that process.

Third, the case count is growing rapidly and a lot of the tests are coming back positive for Ebola, suggesting the outbreak may be far bigger than what’s been detected.

Bhadelia is particularly concerned that some of the patients who have passed away were healthcare workers. “They’re like canaries in the coal mine. It tells you that a lot of patients are being seen who are Ebola patients that are not being diagnosed,” she says. “I think [the current case counts] are the tip of the iceberg.”

Fourth, the area where this outbreak originated has many characteristics that can fuel the spread of disease. It’s remote and lacks good health care infrastructure. It’s a mining area with lots of migrant workers as well as international companies with workers who may travel frequently. And, it’s been plagued by conflict.

“It’s hard to do contact tracing when there’s a lot of people with AK-47s around,” says Bausch. He says the typical public health measures work well “but employing them in this setting is not simple.”

This has experts worried this outbreak will not be stamped out quickly: “My expectation is that we’re not going to be able to bring this outbreak to a close for quite a few weeks, if not months,” says Bhadelia.

How concerned should people be outside of the region?

Karan says he’s worried that cases could pop up in other countries and “I actually don’t think that the United States is really prepared to have a number of people coming back and quarantining here,” he says, pointing out that a specialized quarantine facility in Nebraska is already dealing with people who may have been exposed to hantavirus on a cruise ship.

But for the general public, Bausch isn’t too concerned.

Since the virus does not transmit through the air and because patients only really spread the virus when symptomatic, he says caretakers — at home or in the hospital — are the ones at most risk. Even then, “you never have a situation [in the U.S.] where the healthcare workers say, ‘Well, we don’t have any gloves here. There’s no running water. There’s no soap,” he says.

He acknowledges that there could be some cases but “we’re not going to have a big Ebola outbreak in a high-income country.”

So for the friends and family members who have been calling him asking how risky this outbreak is, he has developed a stock answer. “Go get your flu shot and wear your seatbelt when you’re in a car,” he says, because those risks are far greater than their “extremely, extremely small” risk of getting Ebola.