Ebola fear and conspiracy theories are spreading faster than the disease. But even scientists — who have thought very deeply about Ebola and pandemics — are beginning to worry.



What they fear, however, is slightly different from the zombies and airborne Ebola that keeps many of us up at night. I asked them about what it would take for Ebola to spread further in America and around the world. Here's their worst-case scenario:

1) The Ebola outbreak in West Africa keeps growing

In order for Ebola to move around the world, the outbreak needs to continue to grow in West Africa. Cases there need to keep on their exponential ascent. The more people infected with Ebola at the source, the more likely they are to infect other people, and the more likely those people are to travel and spread the disease. It's the mathematics of Ebola, and it's scary.

For the situation to deteriorate in West Africa, efforts to address it need to fail. The unprecedented international response — led by the US — needs to be implemented too slowly, and needs to continue to lag the growth in cases.



And this is possible, at least for the foreseeable future, since the epidemic has already had such a long lead time before the international community intervened. "We have had more Ebola cases in the last two months than the entire history of the infection combined. It's still in the growth phase," says Ashish Jha, director of the Harvard Global Health Institute. "The longer it continues in West Africa, the bigger a chance it's going to get much more global."

2) The virus finds new hosts in urban areas with weak health systems

Traditionally, Ebola was a rare disease, relegated to remote and rural areas in Africa. It affected a couple thousand people since it was discovered in 1976. This year, there are more than 8,000 cases, and the virus has popped up in well-populated areas, which has helped it to find new hosts and move further.



For the Ebola caseload to continue to balloon, people with Ebola need to continue to turn up in densified areas, particularly with under-resourced and weak health systems.



Peter Piot, who helped to discover and name the virus, told the Guardian newspaper that he was concerned in particular about the link between West Africa and India:

...an outbreak in Europe or North America would quickly be brought under control. I am more worried about the many people from India who work in trade or industry in West Africa. It would only take one of them to become infected, travel to India to visit relatives during the virus's incubation period, and then, once he becomes sick, go to a public hospital there. Doctors and nurses in India, too, often don't wear protective gloves. They would immediately become infected and spread the virus.





West Africa also has strong links with China, where the health system can be similarly patchy.

Watch: How the Ebola outbreak spread out of control:

3) Rich countries with strong health systems continue to fumble and botch their Ebola cases

Epidemiologists have been preparing for this kind of nightmare for decades. "People have been talking about this for probably 20 years," said Stephen Morse, director of infectious disease epidemiology at the Columbia University Medical Center. Yet, even though there is more travel, and more people are living in cities, our global health responses still don't reflect this reality, says Morse. "I have been discouraged by the lackadaisical, the tepid, and late response by the global community."

Since the beginning of this outbreak, one of the truisms about Ebola hitting America, or another developed country, has been that it would be detected and stamped out so fast, it wouldn't have a chance to gain a foothold. "It is not a potential of Ebola spreading widely in the US," Centers for Disease Control and Prevention director Tom Frieden told reporters in July. "We have strong systems to find people if there is anyone with Ebola in the US."



Yet, the first Ebola patient in the US — a recently deceased Liberian national who had arrived from Monrovia days before seeking care in a Dallas hospital — was misdiagnosed on his first visit. He was sent back into the community with antibiotics to fight what was believed to be a common viral infection, even though he told hospital staff about his travel history. He also made it through airport screening in Monrovia, days after caring for a woman who died from Ebola. Now, one of the health workers who cared for him has tested positive for the virus.



There was similar fumbling with Europe's first Ebola case. A Spanish nurse contracted the disease after caring for a repatriated priest from Sierra Leone who later died. While harboring the virus, she sat in on a civil service exam with 20,000 others and visited clinics to get help with her early symptoms. But her temperature was thought to be too low to sound the Ebola alarm bells.



"I have become a little less optimistic" "A few weeks ago, if you had asked me 'is Ebola a danger to the US?' I would say the conventional wisdom is 'no,'" says Morse. "I still think that is largely true but I have become a little less optimistic."



These errors are all very human and all too easy to make. Despite the Ebola public-health protocols sent out to doctors, hospitals, and airports, the airport screenings, the warnings to health professionals who have been exposed to quarantine themselves, the warnings to travelers to stay home if they have come into contact with an Ebola patient, Ebola managed to slip through the cracks of the best health systems in the world. These everyday mistakes need to continue to happen for the virus to move further.

4) Rising panic causes us to make irrational — and harmful — choices that exacerbate the epidemic

Following the announcement of increased airport screening efforts for travelers from West Africa, New Yorker writer John Cassidy lamented the political hot potato that Ebola has become:

The measures that the Obama Administration announced on Wednesday mimicked what some Republicans, including Rick Perry, the governor of Texas, had been advocating. At a press conference on Monday, where he unveiled a new task force to combat infectious diseases, Perry called on Washington to introduce "enhanced screening procedures" and create "fully staffed quarantine stations." At this stage, the only big difference between Perry and the White House is that he wants screening to be extended to "all points of entry" to the United States; the new C.D.C. policy is limited to airports in Atlanta, Chicago, New Jersey, New York, and Washington, D.C.

It doesn't matter that Ebola is currently a small threat in America, and that the real focus should be stopping the outbreak in West Africa. If this political theater devolves into, say, a travel ban in West Africa that further isolates the region, the nightmare epidemic will inch closer to reality: the economies of the affected countries will continue to crumble, getting aid to the region will be difficult or impossible, and Ebola will rage on over there, which again, means it's a threat everywhere.

5) The virus mutates to become more virulent

Every scientist worth his weight in citations has said that the chances of Ebola mutating to become airborne are slim. This is because it would require the virus to change how it is transmitted. This would be like HIV or herpes suddenly becoming airborne. It's something viruses don't do, though of course, nothing is impossible when it comes to biology.



What is more probable, however, is that the Ebola virus now circulating is more virulent than previous strains. Peter Jahrling, a chief scientist at the National Institute of Allergy and Infectious Diseases, has a field team in Monrovia, running tests on samples of Ebola from the sick in this epidemic.

This kind of mutation might help explain why Ebola seems to have spread so effectively this year.

"They are telling me that the viral loads are coming up very quickly and really high, higher than they are used to seeing," he says. The more concentrated the virus in people's bodily fluids, the more infectious and easily spread.



Though Jahrling and his team need to continue to study this hypothesis, he says this kind of mutation might help explain why Ebola seems to have spread so effectively this year compared to previous outbreaks. And this is saying a lot from someone who has studied Ebola and other dangerous pathogens for more than 25 years.

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