SILVER SPRING, Maryland — It’s hot and sticky in Silver Spring in June, and the mosquitoes are everywhere, breeding at construction sites, in backyard ponds and in the swampy pools left behind by two weeks of torrential rains.

It seems unthinkable that anyone would be breeding more mosquitoes on purpose, but deep inside the blue-and-yellow Walter Reed Army Institute of Research, Lindsey Garver and her colleagues are breeding them by the thousands and feeding them human blood.

They are Anopheles — the mosquitoes that spread malaria. One of Garver’s jobs is to make sure these insects get infected with the Plasmodium falciparum parasite that causes malaria.

Then they’re letting them bite people.

Most of the volunteers who have been bitten by the mosquitoes have also been vaccinated with a malaria vaccine. The hope is to find a better dose or a better dose regimen that will get better than the 30-40 percent protection the vaccine currently provides.

Containers hold Anopheles mosquitoes being bred for use in malaria vaccine research at the Walter Reed Army Institute for Research in Silver Spring, Maryland. Volunteers press paper cups filled with mosquitoes against their arms in the hope that the mosquitoes will bite them and transmit malaria. Maggie Fox / NBC News

But 38 of them have not been vaccinated. They’re the controls.

The vaccine development team needs for these people to come down with malaria, to show the mosquitoes really did transmit the parasite.

“I am hoping we see 38 people develop infections,” said Dr. James Moon, an Army colonel who’s leading the vaccine study.

And he hopes all 130 volunteers who got vaccines before exposing the tender undersides of their forearms to the hungry mosquitoes will not get infected.

Malaria infects more than 200 million people a year, the World Health Organization says. It killed 445,000 in 2016, mostly children.

The parasites that cause malaria set up a complex life cycle in the human body. While some are easily killed with modern medicines, in many regions the parasites have developed resistance to most of the drugs on the market.

Widespread use of insecticides eradicated malaria from the U.S., but the fight elsewhere has been tougher.

People can take drugs to prevent infection, but malaria is a disease of poverty. Malaria is an everyday threat across much of Africa, Southeast Asia and South America. Most of those susceptible to malaria have almost no access to drugs either to treat it, or to prevent it.

A vaccine would be the best solution. Only one is in use — Mosquirix, or RTS,S. GlaxoSmithKline worked with the PATH Malaria Vaccine Initiative to develop the vaccine, and now the Walter Reed team is testing it to see if they can make it work better.