Nearly 700 million people are infected by diseases carried by mosquitoes every year. Yet, it turns out that you are more likely to spread a disease amongst another human through travel than a mosquito, researchers have found.

A new study reports that humans are up to 1,000 times more likely to spread diseases like malaria, Zika, and yellow fever than mosquitoes, which have often been viewed as the leading reason diseases spread around the world.

Michael Johansson, the lead author of the study and researcher at the Center of Disease Control in Puerto Rico, credits the findings to the vast amounts of human travel on airplanes and how unlikely it was that infected mosquitoes made it to new locations via airplanes. It was more likely that humans were the cause.

“We expected to find this, but we were surprised by the magnitude,” Johansson told Reuters. He emphasized that the study focused on the “worst-case scenarios” of how severely humans spread disease.

“We need to focus on ways to prevent the spread through humans,” Johansson explained.

Researchers found that there was a 25 percent chance a singular mosquito was on a plane. “Overall, the probability that an airplane traveling from a mosquito-heavy area would lead to infection was extremely low” no matter how much mosquito prevention the airplane went through, the report said. Humans were found to be “hundreds of times more likely” to be the reason why diseases spread from one location to the next.

Johannson also stressed that disease-prevention professionals need to find better ways to prevent diseases spreading amongst humans more than insects.

“We’ve had disinsection policies for a long time, which are targeted at mosquito species and agricultural pests,” he said. “But we need to focus on ways to prevent the spread through humans.”

Governments take steps to control disease spreading

In 2009 during an outbreak of the H1N1 flu virus (swine flu), officials from Saudi Arabia put a stress on making sure people traveling to Mecca for Hajj, the biggest annual gathering of humans in the world, were vaccinated. A large congregation of people is the premium place for the spreading of disease, a story in the Telegraph pointed out, so officials requested people traveling from abroad get vaccinated at least two weeks before traveling to Saudi Arabia. The Saudi government also required British Muslims traveling to the country prove they had a seasonal flu vaccination.

At the end of Hajj, fewer than 100 visitors of the estimated three million people in attendance of the five-day gathering contracted H1N1, and there were five fatalities, a study reported. The study also said that the steps taken by the Saudi government before the gathering contributed to the extremely low infection rate during Hajj.

Humans credited with spread of Zika

The origin of the Zika virus epidemic in 2015 and 2016 has been credited to human travel. Researchers believe that a traveler from French Polynesia—an area impacted by the virus in 2014, before the epidemic started a year later—was infected with the virus. That person then flew to Brazil, where mosquitoes picked up the virus and spread it to epidemic proportions. Zika can also be spread through sexual intercourse, and, despite being largely asymptomatic, the virus can cause severe problems with pregnancy.

Humans are the driving force behind diseases traveling around the world.

Zika later spread over Central and South America and eventually to the United States. The person in the U.S. infected with the disease had not been a traveler but was infected after having sexual intercourse with someone who had traveled from an infected area.

Once in the States—Florida specifically—mosquitoes were the main transportation of Zika. A study published in June 2017 by Harvard Medical School researcher Moritz Kraemer found that 99.8 percent of more than 24,000 mosquitoes in the Miami area tested positive for Zika.

tiger mosquito

Despite the massive quantities of infected mosquitoes, Kraemer agrees that humans are the driving force behind diseases traveling around the world.

“There may be a misperception about how pathogens disperse globally,” Kraemer told Reuters. “With the increase in travel globally, infectious diseases spread quickly from one location to another.”

The findings of Johannson’s study come amidst a statewide effort in California to reduce the mosquito population. Scientists plan to release 20 million sterile mosquitoes into the wild. When the male mosquitoes mate with female mosquitoes, the females won’t hatch any offspring, thus hopefully reducing the overall population over time, according to an NPR report.

“It’s kind of contrary to what a person would think. ‘What, you’re doing what? You’re releasing mosquitoes to control mosquitoes?’,” Steve Mulligan, a leader of the initiative, told the Washington Post. “We are releasing male mosquitoes because male mosquitoes do not bite and cannot transmit disease.”

There may be hope for greater mosquito control after all.

To learn more about mosquitoes and how to get rid of them, read our comprehensive guide.