Scientists love patterns.
It's what makes science possible — and powerful — especially when it comes to infectious diseases.
Over the past 30 years, scientists have noticed a distinctive pattern of mosquito-borne diseases in the Western Hemisphere: Three viruses have cropped up, caused small outbreaks and then one day — poof! — they hit a city and spread like gangbusters.
All three viruses are carried by the same mosquito, called Aedes aegypti. All three have caused millions of cases in Latin America and the Caribbean. And all three have gotten a foothold in the U.S., causing small outbreaks.
Now there's a fourth one lurking in the Brazilian rain forest, says Dr. Anthony Fauci of the National Institutes of Health. It's familiar. And it's deadly: yellow fever.
In a recent commentary for the New England Journal of Medicine, Fauci and his colleague Dr. Catherine Paules explain the pattern seen across Latin America and how the historical information could help us intercept the next epidemic.
The waves of epidemics from the three other viruses started in the 1990s.
First dengue — a nasty virus that can cause hemorrhaging — re-emerged in parts of Latin America after it had been eliminated in 18 countries.
Next up came chikungunya. The virus first appeared in the Caribbean in 2013. It hopped around the islands for a few months and then finally hit the mainland of Central and South Americas, where it caused debilitating joint pain in thousands of people.
Then last year, Zika emerged as the first mosquito-borne virus that can cause birth defects. Still today, the U.S. is seeing about 30 to 40 Zika cases in pregnant women each week.
"Now all of a sudden you start to see this very interesting clustering of yellow fever cases in Brazil," says Fauci.
The outbreak started in December and has swelled to about 600 confirmed cases and more than a thousand suspected cases, the Brazil Ministry of Health reports. Symptoms can include fever, nausea and muscle aches. In about 15 percent of cases, the disease progresses into a toxic phase, which can include jaundice, bleeding and organ failure. There have been about 200 deaths in Brazil.
"That's a potential threat," Fauci says.
So far, the disease is still isolated to a rural area, Fauci says. And it's spreading only among mosquitoes that live in the forest and not in mosquitoes that thrive in cities, called Aedes aegypti.
But that scenario could change quickly, Fauci says, if Aedes aegypti picks up the virus from infected people.
"If Aedes aegypti mosquitoes start spreading yellow fever in Brazil, there's a possibility that you might have an outbreak in very populous areas in Brazil, such as Rio de Janeiro and Sao Paulo," Fauci says.
"Whether that's going to happen, I don't know. But if it does, we're going to get a lot of travel-related cases in the U.S.," he says, "which means physicians here have to be aware of it."
And that's why Fauci penned the commentary: To alert public health officials and doctors so they won't miss cases.
"This is a wake-up call," Fauci says. "Be careful. If someone comes in with an illness that's compatible with the yellow fever, you might want to ask them, 'Have you traveled to this part of Brazil?' "
But there's another reason to keep an eye on yellow fever in the Americas. Unlike Zika, chikungunya and dengue, the world actually has an effective way to prepare for an outbreak. We have a vaccine that is 99 percent effective.
"That's really an amazing asset," says biologist Erin Mordecai, who studies infectious diseases at Stanford University.
"So this is a great example of a time that we could be proactive," she says. "We have a good vaccine, and we need to make sure that there's enough available in the case of a large outbreak."
And right now, that's a bit of a problem, Fauci says. The world's supply of the yellow fever vaccine is low. There aren't enough doses to protect Brazil's population of 200 million, not to mention the rest of Latin America.
"We don't have enough vaccine. Period," he says. "We're going to have to make more vaccine. And that will take time."
In the meantime, predictions that Brazil's outbreak would burn out quickly have turned out to be wrong. The outbreak continues to grow while health officials make deep cuts into the vaccine stockpile.