Along hundreds of miles of Lake Victoria’s shoreline in Kenya, a squadron of young scientists and an army of volunteers are waging an all-out war on a creature that threatens the health of more people than any other on earth: the mosquito.They are testing new insecticides and ingenious new ways to deliver them. They are peering in windows at night, watching for the mosquitoes that home in on sleeping people. They are collecting blood — from babies, from moto-taxi drivers, from goat herders and from their goats — to track the parasites the mosquitoes carry.But Eric Ochomo, the entomologist leading this effort on the front lines of global public health, stood recently in the swampy grass, laptop in hand, and acknowledged a grim reality: “It seems as though the mosquitoes are winning.”Less than a decade ago, it was the humans who appeared to have gained the clear edge in the fight — more than a century old — against the mosquito. But over the past few years, that progress has not only stalled, it has reversed.The insecticides used since the 1970s, to spray in houses and on bed nets to protect sleeping children, have become far less effective; mosquitoes have evolved to survive them. After declining to a historic low in 2015, malaria cases and deaths are rising.Climate change has brought mosquitoes carrying viruses that cause dengue and chikungunya, excruciating and sometimes deadly fevers, to places where they have never been found before. Once a purely tropical disease, dengue is now being transmitted in Florida and France. This past summer, the United States saw its first locally transmitted cases of malaria in 20 years, with nine cases reported, in Texas, Florida and Maryland.“The situation has become challenging in new ways in places that have historically had these mosquitoes, and also at the same time other places are going to face new threats because of climate and environmental factors,” Dr. Ochomo said.At the Kenya Medical Research Institute in Kisumu, workers separate male from female mosquitoes trapped overnight at study sites in the surrounding region. A trapped mosquito at the research institute. Only female mosquitoes bite humans, and thus transmit malaria.Technician Celestine Akinyi Ogutu takes mosquitoes from a freezer. The trapped insects are frozen to kill them, then dissected to study whether they have bitten a human and carry the malaria parasite.Scientists around the world are pressing hard for new solutions, including novel technologies that Dr. Ochomo is testing. They have developed some promising approaches, including a new generation of tools that modifies mosquitoes biologically, and genetically, to block disease.But such efforts have been stymied by cost and regulatory hurdles. The process for getting any of these tools to the places where children fall ill with each new season of rain involves years of testing and regulatory reviews that are painfully slow and badly underfunded.“It’s just ridiculous how much time we are wasting before we can get into the field and actually start saving lives,” said Bart Knols, a Dutch vector biologist who runs mosquito-borne disease elimination projects across Africa, Asia and the Caribbean.The biggest killerMalaria has killed more people than any other disease over the course of human history. Until this century, the battle against the parasite was badly one-sided. Then, between 2000 and 2015, malaria cases dropped by a third worldwide, and mortality decreased by nearly half, because of widespread use of insecticide inside homes, insecticide-coated bed nets and better treatments. Clinical trials showed promise for malaria vaccines that might protect the children who make up the bulk of malaria deaths.That success lured new investment and talk of wiping the disease out all together.But malaria deaths, which fell to a historic low of about 575,000 in 2019, rose significantly over the next two years and stood at 620,000 in 2021, the last year for which there is global data.There were more cases of dengue in Latin America in the first half of this year — more than three million — than in all of 2022. Bangladesh is in the grip of its largest-ever dengue outbreak, with 120,000 cases by the end of August. Cases of and deaths from chikungunya and other mosquito-borne infections have started rising, too, in many regions of the world.Entomologists at the Kenya Medical Research Institute testing anti-mosquito technologies in a model village inside a giant mesh cage.Augustine Ochieng, a volunteer in Alego, western Kenya, tracked data on a tablet provided to him by the research institute. He was asked to sit with his bare legs exposed all night, harvest any mosquitoes that try to bite him and log the results.One of the main reasons is that mosquitoes are highly adaptable. As more and more people are protected by nets or sprays at home, mosquitoes have begun to bite more outdoors and in the daytime, instead of indoors and at night, the historic pattern of the malaria vector species in Africa. Because the genetic makeup of mosquitoes evolves quickly in response to changing environmental conditions, they have also developed resistance to the class of insecticides in wide use — while the malaria parasite itself is increasingly resistant to the once highly-effective drugs used to treat it.And a new mosquito that thrives in urban areas has come from Asia to Africa, where the spread of malaria had always been confined largely to the countryside. That change has made more than 100 million additional people vulnerable to mosquito-borne infections, researchers at the University of Oxford recently estimated.The multiplying risks, experts say, mean there’s an urgent need for a method to protect people from all mosquitoes — one that will help defend against malaria, but also dengue, yellow fever and whatever pathogen lurks around the corner. (Only female mosquitoes bite; they need the protein in blood to produce eggs.)But it takes a decade or more to design, develop, test and produce a new technology or intervention. Contrast that with the six-week life spans of mosquitoes, which are constantly evolving to elude the ways we try to kill them.The bulk of the money for these efforts to date has come from high-income nations and private philanthropists, but funding levels have plateaued. Multiple researchers said it was increasingly difficult to motivate the kind of investment they need for large-scale trials of new methods.“Sooner or later, funding bodies are going to divert that money to other things,” Dr. Knols said. “They’re going to say, ‘We’re putting it into agriculture, or into schooling.’”A new problemAmukura in Busia County, Kenya, where the entomologist Eric Ochomo oversees two large clinical trials of new tools to fight mosquitoes.In the towns and villages of Busia County in Kenya, the roads begin to fill while the sky is still the streaky purple of dawn, with farmers on the way to their fields, children in freshly pressed uniforms walking to school and moto-taxi drivers reporting to the market.Dr. Ochomo’s research has found that the mosquito Anopheles funestus is feasting on them: The species, once thought to bite mostly sleepers in their beds at night, now bites outside in the daytime.Four in 10 people on these red-dirt roads are carrying the malaria parasite, even if they don’t have symptoms, studies by Dr. Ochomo and his colleagues have found. Some outdoor and daytime biting was likely happening all along, but no one was really tracking it because the focus was on the vulnerable sleepers.Twenty years ago, in the early days of the mass distribution of bed nets, malaria case rates plunged immediately, and there was an optimism that the nets might be enough, said Audrey Lenhart, the chief of entomology at the U.S. Centers for Disease Control and Prevention. Instead, she said, they have helped to create a new problem.“Think about it: You put bed nets everywhere, then the mosquitoes that bite people inside, they’re going to die out,” Dr. Lenhart explained. “The ones that are going to survive are the ones that are biting people sitting outside, biting livestock, the ones that aren’t in the houses, right? So then those are the ones that are reproducing and keeping the population of mosquitoes there.”Processing blood samples collected from children at the Amukura Health Centre as part of a clinical trial of a new mosquito-control method. Microscope slides with blood collected from children enrolled in the clinical trial.Insecticides that don’t workMost of the current insecticides in use are pyrethroids, which were developed in the 1970s and derived from the chemical compounds in an ancient mosquito deterrent made by crushing aster flowers. They have been used for everything, including bed nets and for spraying on walls.With mosquitoes around the world now highly resistant to them, there is an urgent search for something new.In 2005, the Bill & Melinda Gates Foundation invested $50 million in a project called the Innovative Vector Control Consortium to search for effective insecticide compounds. The consortium asked large agrochemical companies to look in their chemical libraries for molecules that might affect mosquitoes in novel ways and be safe and durable enough.“We started with four-and-a-half-million compounds, and we’re down to four,” said Nick Hamon, who recently retired as the consortium’s chief executive.To work as a new insecticide, compounds have to be safe for humans, come in solid form and not be water soluble. And they have to kill mosquitoes in a substantively different way than pyrethroids do, because mosquitoes develop resistance not just to an individual chemical, but to the way the chemical kills them.Companies must now complete the safety and testing process on the only four compounds that tick all those boxes. It is intensely costly and slow, said Susanne Stutz, the chief chemist at the German chemical company BASF.“It’s always a race with the mosquito, who is faster: Usually, the mosquito wins because it develops the resistance much faster than new products come out,” she said.Dr. Ochomo inspecting a pond near a home in Asembo, a village on the coast of Lake Victoria, for larvae from malarial mosquitoes.The edge of Lake Victoria in Siaya County, western Kenya, the area of the mosquito trials.Hanging up spatial repellent panels in a home. The panels are intended to protect people from mosquito bites when they are awake rather than when they’re sleeping in bed.The World Health Organization requires two large randomized clinical trials, carried out in two different geographic and epidemiological settings, showing a significant positive impact on public health in both, before it will recommend use of a new intervention against mosquitoes. The organization says the policy is designed to make sure that countries with limited means are making the best decisions about where to spend their money and to ensure that products are backed by rigorous evidence. Yet the world’s multiplying mosquito problems increasingly require solutions tailored to specific situations: What works to protect children in the African Sahel won’t be what works to protect loggers in forests in Cambodia.Dr. Ochomo is the principal investigator on two large randomized clinical trials of mosquito interventions. In one $33 million project, researchers are testing the effectiveness of spatial repellents — squares of plastic film that can be hung on the walls inside homes and that dispense low doses of a chemical that confuses mosquitoes and prevents them from biting — in both dengue and malaria risk areas.S.C. Johnson & Son Inc., the Wisconsin-based company that developed the spatial repellent being tried in Kenya, has donated millions of dollars in products for testing. Such largess is unusual — and not a sustainable pathway for vector control research, said John Grieco, a professor of biological sciences at the University of Notre Dame who coordinates the multicountry spatial repellent trial, which is also running in countries including Mali and Sri Lanka.The spatial repellents and most other new tools are commodities: items that have to be bought, then bought again six months or a year later. The protection they offer is temporary, as is the funding that allows for their purchase.The Gates Foundation, the major backer of most of the clinical trials of the commodities, has also had to cover most of the cost of BASF’s testing of new chemicals for use on bed nets, because there is not sufficient profit incentive for a private firm to do it, Dr. Stutz said.“How do you keep the companies that know how to innovate in this space?” Dr. Hamon asked. The Innovative Vector Control Consortium lost one of its key industry partners in 2017.Beryle Etyang prepared to deliver a cooler of spatial repellent panels via motorbike from a refrigerated storage facility at a Kenya Medical Research Institute site in Alupe.“They just said, ‘We can make more money somewhere else,’” Dr. Hamon said.Some experts believe the emergence of dengue fever, and now malaria, in middle- and high-income countries could generate new funding because it creates a wealthier market that may spur new corporate investment.Skeptics in the entomology world look at the race for new commodities, and suggest it ignores a lesson from history: They say that only the same strategies that high-income nations used more than a century ago will once again give humans the edge over mosquitoes. In southern Italy and the American South in the early 1900s, and in Brazil in the 1950s, it was large-scale environmental management that made a difference, said Silas Majambere, a Burundian vector ecologist who has worked all over Africa and Asia.That means draining breeding sites, spraying larvicides (which are biological toxins and don’t cause resistance) on water that can’t be drained and moving homes out of swampy areas. Those steps won’t help combat urban mosquitoes. To protect against them, people need screened windows and solid roofs: better houses.“If we had spent the last 40 years doing these things, with the same budget, where would we be today?” Dr. Majambere said.Hope and challengesMary Oketeti, 55, said her 12-year-old daughter Cynthia had to be hospitalized three times last year with malaria.Sirista Etyeng, 78, said she falls ill with malaria five or six times a year. She was given a spatial repellent in her home as part of a randomized controlled trial.While malaria cases are far lower than they were 20 years ago in the Busia area, the stalled progress means the disease continues to erode family health, income and futures.“When there is a malaria case in the house, it suspends life,” said Mary Oketeti, a farmer who lives about an hour’s drive outside the town of Busia. She gets malaria three times a year, and her 12-year-old daughter twice as often as that. The family then has to spend what is needed for treatment.“If there’s a chicken in the house you sell it,” she said.A chicken might be worth 600 Kenyan shillings, or $5; a trip to the medical clinic, with transport, a diagnostic test and drugs for malaria, will cost at least that much. Repeated bouts of malaria keep children out of school and adults from working; they wipe out savings. Ms. Oketeti said she must stay home from the fields she farms to care for a sick family member for a few days every month.Dr. Ochomo and his team recently received data from the midpoint of the clinical trial of spatial repellents. Malaria cases were significantly lower in families that had them compared to those that had devices that used placebo repellents. If that trend holds, the next challenge will be convincing the W.H.O. to endorse the use of spatial repellents, then the Kenyan government to buy them.It won’t be hard to convince people in Busia to use them, though, he said.“People already know that nets are not enough, they need something more, and they’re happy to see us,” he said. “They say, ‘Finally, someone is coming to try to help with this.’”

The stories the mothers tell when they gather at the Awendo Health Centre in western Kenya are a catalog of small failures, missed opportunities and devastating consequences. What unites the two dozen or so women who meet periodically, on wooden benches in a bare clinic room or under a tree in the courtyard, is their children: All have H.I.V.It has been two decades since efforts to prevent the transmission of H.I.V., the virus that causes AIDS, from mother to child during pregnancy and birth began in earnest in sub-Saharan Africa. Yet some 130,000 babies are still becoming infected each year because of logistical problems, such as drug shortages, and more pernicious ones, such as the stigma that makes women afraid to seek tests or treatment.Then, many of the children who contract the virus are failed a second time: While the effort to put adults on H.I.V. treatment has been a major success across the region, many children’s infections are undetected and untreated.Seventy-six percent of adults living with H.I.V. are on treatment in sub-Saharan Africa, according to U.N.AIDS, a United Nations program. But just half of children are.An estimated 99,000 children in sub-Saharan Africa died of AIDS-related causes in 2021, the last year for which there is data. Another 2.4 million children and adolescents in the region are living with the virus, but just over half have been diagnosed. AIDS is the top cause of mortality for adolescents in 12 countries in East and Southern Africa.“The focus for a decade in the global AIDS response has been controlling the epidemic, and it’s amazing that treatment has reached so many adults,” said Anurita Bains, who heads global H.I.V./AIDS programs for UNICEF. “But children aren’t going to spread H.I.V., so they dropped down the priority list. They’ve been almost forgotten.”She added: “Children with H.I.V. are harder to find than adults, we have fewer tools to test and treat them, and they rely on their caregivers to access health care.”Migori, a town in western Kenya, has one of the highest rates of H.I.V. prevalence in the country. Many clinics here haven’t had H.I.V. tests to give pregnant women for several years.Preventing a woman from passing H.I.V. to a child at birth is, in theory, relatively straightforward. The national policy in every sub-Saharan African country with a high prevalence of H.I.V. stipulates that all pregnant women are to be screened for the virus and that those who test positive should start treatment right away.To catch any missed cases, women are supposed to be tested again when they are in labor. If they are positive and not on treatment, they are to be given drugs to block transmission. Their babies should be given another drug for the first six weeks of life. In more than 90 percent of cases, this protocol is enough to prevent a child from becoming infected. A mother on H.I.V. treatment has a low risk of infecting a child while breastfeeding.The Fight Against H.I.V.An estimated 40 million people are living with H.I.V. worldwide. About 10 million of them do not have access to treatment.A New Shot: An injection every two months rather than a daily pill could shield many more women from H.I.V., but the shot is unavailable in places that need it most.Pandemic Setbacks: Before Covid-19, the world had been making strides against global illnesses like H.I.V. The pandemic has changed that for the worse.A Visionary: Ravindra Gupta led the efforts that resulted in the second case of a patient being cured of H.I.V. Then he was drawn into Covid research.A Promising Treatment: In February, researchers announced that a woman became the third person ever to be cured of H.I.V. thanks to a new transplant method that could help more people from racially diverse backgrounds.But progress has flatlined in several countries in the last five years, and the Covid pandemic set it back further, with disruptions to the supply of tests and drugs, clinic shutdowns, staff shortages and a shifting away of attention to the fight against AIDS.“It’s very painful when you are with a pregnant woman who is almost delivering and there is no medication and you wonder, Will the child be positive or not?” said Caroline Opole, who is a volunteer “mentor mother,” counseling women who test positive for H.I.V. at prenatal testing as she did.A pharmacist packed a bottle of antiviral pills at the pharmacy at the Ogwedhi Health Centre in Migori.The stories from the mothers at the Awendo clinic underscore the routine failings seen across the health system here: The clinic was out of tests. The clinic was out of drugs. The lone overworked nurse was too busy to deliver a vital dose of medication when a woman was in labor.“Prevention of mother-to-child transmission, whereas there has been a lot of effort to scale it up, has not performed as well as we should have done,” said Dr. Andrew Mulwa, director of medical services for the national health ministry in Nairobi.Laurie Gulaid, UNICEF’s Nairobi-based regional adviser on H.I.V./AIDS, said the problem here in Kenya and beyond was the gulf between written policy and what the government actually funds, makes a priority and puts into practice in primary health centers such as Awendo.“The intentions are good, but the infrastructure, the resources, the training, the staff — those aren’t there yet, not the way they need to be,” she said.In Migori, a county in the region that has one of the highest rates of H.I.V. prevalence in Kenya, many public clinics haven’t had H.I.V. tests to give pregnant women for several years. Depending on whom you ask, this is because of supply chain disruptions, disputes with donors or poor planning by officials. If women know they have H.I.V., then sometimes their babies are on antiretroviral medication. But sometimes those pediatric drugs are out of stock, too.Ms. Bains of UNICEF said countries needed to redouble their commitment to children. “We need to find the kids we’ve missed, test them, get them on to treatment,” she said. “We need resources to do this, but it also requires robust health care systems and capacity — nurses in clinics and community workers supporting mothers.”Joyce Achieng, left, with her children and others at her home in a village outside Migori. She has struggled to get treatment for two who have H.I.V.Closing the treatment gap for children will also take political will, she added. “When international funding is being allocated to a country, we need to always ask, How will the money be used to reach and support children living with H.I.V.?”But even when the drugs are available, it is not always as simple as taking them, as Joyce Achieng knows. Ms. Achieng was not screened for H.I.V. when pregnant with her first two children, now 12 and 10. She learned she was infected after being tested while pregnant with her third, a girl who is now 7.But a woman in this region is accused of infidelity if she tests positive, Ms. Achieng said, and she feared she might be assaulted or driven from her home if she told her husband.At the time, her husband was working in another part of the country, so she could begin H.I.V. treatment and give the drugs to the baby after her birth while keeping the news to herself. Her daughter tested negative for H.I.V. at age 2. When the clinic encouraged Ms. Achieng to bring her other children in for testing, she did and learned they were negative, too.A year later, she became pregnant again, but this time her husband was home. She couldn’t always hide the drugs she needed for herself or the new baby, another girl. It was hard to come up with excuses to walk the eight kilometers to the clinic to pick up medications or a reason that she needed 100 shillings (about a dollar) for a motorbike taxi. So neither she nor the baby took the drugs consistently, and the infant tested positive for H.I.V. at 6 weeks old.“I cried for the longest time,” Ms. Achieng said. The nurse who gave her the news urged her to start treatment for herself and her daughter again, but she was overwhelmed with guilt and despair.“I said, What is the use, if I have made my own child sick?”Nancy Adhiambo, left, and her children outside her home in Ogwedhi in Migori County, Kenya.Eventually, some tenacious medical staff and volunteers helped her tell her husband she had H.I.V. and to resume treatment. Today, Ms. Achieng is fit and cheerful, and her daughter runs into the house after school to show her a page where she has labeled and colored fruits and shapes. She giggles softly when her older brother takes her for rides in a wheelbarrow.Her daughter takes a pediatric formulation of a drug called Dolutegravir. A highly effective antiretroviral medicine, it recently became available as a strawberry-flavored syrup, which spares parents from battling to get small children to swallow pills each day.“The new drugs do wonders,” said Tom Kondiek, the pediatric clinical officer at the main public hospital in Migori. “Children who are on their deathbed, you start them on medication and then you see them very active and you would not even know they are suffering from H.I.V.”But to start them on medication, health workers must know the children have the virus, and that is where the system breaks down, he said. They may be brought to a clinic over and over but never be tested because staff don’t think of it for a child of 4 or 5, or because there are no tests available.Even when individual women are diagnosed and connected with treatment, health systems too often fail to think of their families, Ms. Gulaid said. In routine care, children are typically seen at 6 weeks old for immunizations and a nutrition screening, but H.I.V. tests are included only for babies known to have been exposed. Other children may not be seen again unless they fall very sick, and it is not standard practice to test all children, the way the clinic did with Ms. Achieng’s.Ms. Adhiambo, picking up her antiretroviral drugs from the Ogwedhi Health Center, chatted with Sheila Abuyu, who oversees H.I.V. care there.Nancy Adhiambo, a mother of five, learned she had H.I.V. during her third pregnancy. She started treatment but struggled to stay on the drugs as she moved around while leaving a chaotic relationship, and she couldn’t obtain medication consistently for her baby.That little girl, who is now 8, wasn’t tested for H.I.V. for years, even though she was often sick with pneumonia as a toddler. It wasn’t until last year, when Ms. Adhiambo found herself living down the street from a clinic in Migori town and joined a tightknit mothers’ group that she had all her children tested and learned that her third child was infected. So was her last-born, a 1-year-old. (Her two oldest and her fourth child were negative.)These days, the older daughter’s H.I.V. is well controlled, and so is Ms. Adhiambo’s. Her face folded into a pleased half-smile when the clinic director congratulated her on the girl’s low viral count.But when Ms. Adhiambo stopped by the pharmacy for the children’s drugs, she heard the same answer she had been given for weeks: The free pills were out of stock. She couldn’t afford the ones that were for sale in town, given that she earns at most a thousand shillings, about 10 U.S. dollars, each month as a hairdresser, she said, so she would divide her remaining tablets among the children.“Poverty complicates things,” she said bluntly. “We can only hope for the best.”