Human viruses are like a fine chocolate truffle: It takes only one to get the full experience.
At least, that's what scientists thought a few days ago. Now a new study published Thursday is making researchers rethink how some viruses could infect animals.
A team at the U.S. Army Medical Research Institute of Infectious Diseases has found a mosquito virus that's broken up into pieces. And the mosquito needs to catch several of the pieces to get an infection.
"It's the most bizarre thing," says Edward Holmes, a virologist at the University of Sydney, who wasn't involved in the study. It's like the virus is dismembered, he says.
"If you compare it to the human body, it's like a person would have their legs, trunk and arms all in different places," Holmes says. "Then all the pieces come together in some way to work as one single virus. I don't think anything else in nature moves this way."
Most viruses have simple architecture. They have a few genes — say about a half-dozen or so — that are packaged up into a little ball, 1/500th the width of a human hair.
"You can think of it like a teeny-weeny tennis ball with spikes," Holmes says.
When the virus infects a cell, the ball latches onto the cell's surface, opens up and pops its genes into the cell.
Poof! The cell is infected. That's all it takes. One ball, sticking to one cell.
But that's not the case for the Guaico Culex virus. It has five genes. And each one gets stuffed into a separate ball. Imagine five tennis balls, each with a different color: a red tennis ball, a blue one, a green one, a yellow one and an orange one.
Then to get infected with the virus, a mosquito needs to catch at least four different colored balls, researchers write in the journal Cell Host & Microbe. Otherwise the infection fails.
"The fifth ball seems to be optional," says Jason Ladner, a genomicist at USAMRIID, who helped discover the virus. Getting the fifth one could control how dangerous the virus is, he says.
Ladner and his team found the virus inside a Culex mosquito found in Guaico, Trinidad — hence the name of the virus, Guaico Culex. Culex mosquitoes are common across the U.S. and spread West Nile Virus.
The study is part of a larger project aimed at figuring out what viruses, in addition to Zika and yellow fever, could be lurking inside mosquitoes and possibly waiting to spill over into people.
"Teams are going out all over the world, collecting mosquitoes and seeing what viruses are there," Ladner says. The goal is to learn about these viruses before they become a problem.
"We're trying to make sure that we're not blindsided when the next virus comes around," virologist Gustavo Palacios, who help lead the study, wrote in a statement. "With all of the diversity seen in these emerging viruses, we never know what the next one will be to have an impact on human health."
So far, it doesn't look like Culex Guaico can infect people. But Ladner and his colleagues at the University of Wisconsin found a related virus in a rare species of monkey found in Uganda, called red colobus.
Indeed, each year, scientists are finding thousands of new viruses, says Vincent Racaniello, at Columbia University. "It's hard to put a number on it. But it's huge."
"We finally have the tools to find them," he says.
But that doesn't mean we can immediately understand what they do, or even whom they infect.
"There's so much we don't know about viruses," Racaniello adds. And with viruses, really anything is possible. "We should always expect the unexpected," he says.