Right now, yellow fever is causing an epidemic in South America, and dengue has been increasing in Central America. But in the United States, the most alarming disease linked to mosquitoes is Zika, which can cause devastating birth defects.

Zika has been a persistent concern since January 2016, when a Houston man became the mainland United States’ first case, arriving back from a trip to El Salvador with the fever, rash and red eyes of full-blown infection. Now more than 5,200 U.S. residents have come down with the virus, at least one in every state except Alaska. The vast majority were infected by being bitten outside the country, and a small number by having sex with someone who was infected that way. But more than 220 people have caught Zika from local mosquitoes carrying the virus. Almost all of those victims live near Miami, and six live in Brownsville, Tex., along the Mexican border. No one can say yet whether those clusters are random blips or early indications of a pattern of transmission that will blow up into an epidemic when the weather warms this year.

Aedes aegypti are present in more than half the states, from California to Florida and as far-flung as San Francisco, Kansas City and New Haven; entomologists have found that they regularly survive through the winter in sheltered spots in Washington, D.C. Unlike the salt-marsh mosquitoes that whine through beach towns at twilight or the night-biting Culex that carry West Nile between birds and humans, aegypti prefer proximity to people; we are their favorite meal. To get to us, they fly into houses and conceal themselves in closets and under beds and furniture. They have evolved to breed in the tiny pools of water we carelessly create around us: in an abandoned tire, the saucer under a houseplant, even an upturned bottle cap.

Like West Nile, Zika can cause high fevers and paralysis — but unlike West Nile, it can also trigger catastrophic birth defects in the children of women infected while they are pregnant. It appears to destroy brain tissue while a fetus is growing, causing the skull to collapse. It also seems to cause brain damage, and eye, ear and joint abnormalities later on — though what will happen to babies as they grow is uncertain, because all the children born to Zika-infected mothers are still toddlers. In the United States, the C.D.C. has identified that 1,311 women who were pregnant in the past year were possibly infected with Zika; 56 of their children were born with Zika-related birth defects. In seven cases, the pregnancies ended early, and the fetuses were shown to have been affected. The C.D.C. recently announced that nearly 10 percent of women infected while they were pregnant had a child with a birth defect — 15 percent if they were infected in their first trimester.

The combination of an ugly virus and a stealthy predator is unnerving — especially because in the year since that first Houston case, it has become clear that the United States is more vulnerable to Zika than anyone thought. Like generals basing their strategy on the last war they fought, public-health experts have set up their defenses based on what worked for previous threats. The traps that health departments bought to catch Culex mosquitoes are not attractive to Aedes. The spraying with pesticide by trucks and airplanes that knocks down nuisance mosquitoes cannot reach ones that have sneaked into buildings. The best defense against Aedes mosquitoes turns out to be not big municipal gestures but small individual actions: destroying their habitat by emptying the pools of water where they reproduce, and keeping them from eating by repairing windows screens and wearing bug repellent.

Those strategies require that landlords and municipal authorities pay attention to housing repairs and garbage pickup, and ask families who probably have other priorities to stay alert to conditions they did not cause. Since the start of the Zika epidemic, a few disease experts have been warning that the parts of the United States where Aedes mosquitoes flourish — the Gulf Coast, especially its largest cities — are also those that possess the worst poverty and municipal neglect, and that are particularly vulnerable to an outbreak.