BROWNSVILLE, Texas — How do you capture the deadliest animal on Earth, one that has been responsible for the death of more people than any other species in history? Here at America’s southern border, it’s not with high-tech weaponry, but with a black plastic tub of stinky water tucked under a bush.

Surrounded by tall, yellowing grass near a sports complex, the bait water reeks of damp soil and decaying vegetation. And on top of the basin is the trap: a gray toolbox with a small battery-powered fan sucking air into a mesh net.

This simple, humble device is called a gravid trap and its fetid smell is meant to mimic the ideal breeding site for mosquitoes. On a recent, humid Texas summer day with triple-digits on the thermometer, it proved to be very effective: Ensnared inside are dozens of mosquitoes, many ready to lay eggs.

Collecting mosquitoes from traps and looking for larvae in roadside ditches isn’t an activity you’ll find in South Texas tourism brochures. But for Brownsville officials like Yaziri Gonzalez, 32, it’s one of the more important — and enjoyable — parts of the job of understanding the extent of mosquito-borne threats. To lay eggs, mosquitoes have to drink blood, so if they can be captured right before they breed, she can find out what they’ve been biting, and potentially, what diseases they’re spreading.

The diseases these insects may carry have left an indelible mark on history and culture, here in the US and in the world. Malaria has been around for as long as people walked on two feet, and it still infects around 250 million people per year and kills more than 600,000. Yellow fever infects about 200,000 people around the world annually, causing around 30,000 deaths.

These diseases prompted the creation (and popularity) of gin and tonic cocktails, shaped the construction of the Panama Canal, and even influenced the outcome of the Civil War. Mosquito-borne diseases like malaria used to infect tens of thousands of people in the US each year, and in regions like the Tennessee River Valley, almost one-third of residents were infected. Mosquitoes are humanity’s blood tax collectors.

Right now, most of the devastation from infections transmitted by mosquitoes is in developing countries. In much of the US, mosquitoes are mostly just a buzzy, itchy nuisance.

It took decades of painstaking public health work in the US to control disease-carrying insects, reshaping the landscape and using blunt tactics like draining swamps, cutting down forests, and recklessly spraying pesticides like DDT, whose effects we’re still dealing with.

Continuing to keep these diseases away demands constant vigilance, but the country’s success in diminishing mosquito-borne disease has pushed surveillance down the list of public health priorities, and few health departments are investing the resources to keep tabs on these bloodsuckers. Laying traps, collecting them a day or two later, cataloging the insects, running genetic tests to identify pathogens, collating the data, and preparing a counteroffensive is difficult, tedious work, and Gonzalez is one of just a handful of experts in the whole country doing this job full time, and Brownsville is one of the few cities willing to invest in her work.

That’s because Brownsville, a city of 190,000 people on the US-Mexico border, can’t afford to be complacent.

Every day, thousands of people — potential hosts — cross back and forth by car and on foot from Brownsville and the Mexican city of Matamoros to go to school, to work, or to the doctor, in addition to unauthorized border crossings. For some residents, the towering border fence is their backyard fence.

Brownsville’s location has made it a sentinel for the US-wide rise in vector-borne disease driven by travel, urbanization, and a warming climate — and the city’s efforts to stay........

© Vox