There are more than 200 species of mosquitoes in the United States, but only a handful are known to be dangerous to humans because of the diseases they carry.

When the conditions are right, swarms of these bloodsuckers can fill the air, ruining picnics, ball games, and campsites. But now, the concern for mosquito-borne disease threats is rising. While a mosquito bite may have long been merely an itchy inconvenience, getting bitten today could be much more serious.

The good news is mosquitoes aren’t indestructible monsters; a well-aimed finger flick can end one forever. Most live just a few weeks and rarely venture more than a few miles from where they’re born. They aren’t fans of the cold — most mosquitoes stop biting when temperatures drop below 50 degrees Fahrenheit. Freezing temperatures can kill off adult mosquitoes, though their eggs can survive until the next warm season.

Most of us in the US haven’t needed to worry too much about vector-borne disease threats. Here, diseases such as malaria and yellow fever have dropped in the rankings as public health threats over the 20th century with concerted efforts to keep them in check.

But this progress is fragile — and it’s time to be on the lookout.

Dangerous mosquitoes are starting to regain ground. Temperatures are rising on average due to climate change. That is opening up new habitats for these bloodsuckers while making them more vivacious in places where they’re already established. In general, the ranges of the mosquito species we need to be most worried about in the United States are expanding northward. Newer invasive mosquito species that arrived as stowaways are also setting up shop across the country. It’s not just the geographic range — warmer temperatures are allowing mosquitoes to survive at higher elevations, too.

Mosquitoes are also very sensitive to humidity and rainfall. They need water to lay their eggs and for their larvae to mature, and the climate is changing in rather unfortunate ways in this regard, too; more extreme rainfall events are creating the standing water conditions that are ideal breeding grounds for some of the more dangerous disease carriers.

Humans have also stacked the deck in mosquitoes’ favor. Deforestation and ecosystem losses mean that there are fewer natural mosquito predators like bats and birds to keep a lid on their numbers, while urbanization — with its more impervious roads, sidewalks, and parking lots — creates more places where water can pool and mosquitoes can breed. International travel is also much more feasible these days, and more people are heading back and forth from places where mosquito-borne diseases are more common. If enough travelers get infected abroad and return home to the US, they can become the seeds for local disease outbreaks as mosquitoes spread........

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