"Adolescents have been entirely absent from national health plans for decades," said Dr. Flavia Bustreo, assistant director-general at the WHO, in a statement. "Relatively small investments focused on adolescents now will not only result in healthy and empowered adults who thrive and contribute positively to their communities, but it will also result in healthier future generations, yielding enormous returns."

Factors such as poor diet, lack of exercise and risky sexual behavior, which can impact health throughout a person's life, also begin during adolescence and need to be taken into account, according to the report.

The importance of adolescent health has been increasing," said Dr. David Ross, a medical officer at the WHO who co-authored the report. "But a lot more needs to be done."

Injuries and infections among teen boys

Though road injuries were the greatest cause of death globally, data showed them to be particularly harmful for boys 15 to 19 years old; most of them were non-drivers, such as pedestrians and cyclists. The second leading cause of death for boys was interpersonal violence, such as physical or sexual abuse, or emotional threats.

"In most deaths by road accident, the adolescent is a passive victim," Ross said. "In only a small proportion are they the driver." That proportion is higher in high-income countries, he added.

Dr. Sonia Saxena, a primary care expert at Imperial College London who was not involved with the report, highlighted that previous studies have found that the leading causes of death among adolescents in higher-income developed countries is risky behavior, such as drug and alcohol abuse, rather than the environment they live in.

"These factors for adolescents are around their whole environment," she said. "It should not be dangerous to walk down the road."

But Saxena highlighted that more detail is needed about why people are dying from injuries, highlighting that underlying factors could vary: being drunk and walking around at night, being hit by an exhausted driver or unsafe streets in general.

Pregnancy risks among girls

The causes of deaths among girls globally also had a clear division based on age, with most of those 10 to 14 dying from lower respiratory infections, such as pneumonia. The report suggests that this stems from their exposure to indoor air pollution, through the use of polluting fuels to cook in their homes.

"They are working in appalling conditions, or maybe there are also not enough services to treat infections in a timely manner," Saxena said.

Older girls, ages 15 to 19, saw greater risk of deaths from pregnancy complications, childbirth or unsafe abortions.

Saxena highlighted the discomforting nature of this latter cause, asking, "Why are 15- to 19-year-old girls getting pregnant at all?" She added that if pregnancy does occur, complications and unsafe abortions should not be commonplace.

Young girls with little family or community support are more likely to find themselves in situations where sex work becomes an option, opening them up to pregnancy and new infections, Saxena said.

Global variation

"It's an impossible task to compare the world," Saxena said, adding that quality of the data available will vary significantly by region.

The picture is very different when leaving global numbers behind to focus on regional causes of death among adolescents.

"While some conditions are common across regions, such as road injuries and suicide, there are others that more prominent in certain regions," Ross said.

Road injuries were the leading cause of death, when combining sexes, in high-income countries as well as the low- and middle-income countries of Southeast Asia and the Western Pacific regions of the WHO. However, self-harm took the lead in Europe, collective violence (war) in the Eastern Mediterranean, violence and abuse in the Americas and lower respiratory infections in the WHO's African region.

"There is huge amounts of interpersonal violence" in the Americas, Ross said, further highlighting the continued burden of infectious diseases in Africa. "That's the only region where HIV is still in the top five."

Other infections dominating as a cause of death in African adolescents include meningitis, diarrheal disease and lower respiratory infections.

"With infectious illness, this is still on the development path," Saxena said, adding that the less developed a country is, the greater the burden infections have on the population.

The influence of conflict

Another area highlighted by the report data is the health of adolescents affected by conflict or humanitarian crises, where young people must take on adult responsibilities at a younger age than expected, such as earning money or caring for younger family members.

As a result, they suffer in terms of schooling, and girls are more likely to take part in sex work or marry earlier than planned.

The report highlights that health impacts stemming from this -- and leading to an increased risk of death -- include malnutrition, pregnancies, sexual violence and mental health issues.

Improving care, education and support

Experts advise that improved health care services, including the provision of better social and mental support, as well as acknowledging the role -- and value -- that families and communities have to play, will help bring these numbers down.

"Parents, families and communities are extremely important, as they have the greatest potential to positively influence adolescent behavior and health," said Dr. Anthony Costello, director of Maternal, Newborn, Child and Adolescent Health at the WHO, in a statement.

"There is very strong evidence that parenting makes a huge difference to adolescent health risk," Ross added.

The report itself recommends a range of strategies, including more comprehensive sex education in schools, mandatory seatbelts and helmets, cleaner cooking fuels indoors and increased drinking age limits.

Ross believes these recommendations will only be possible if those in the health sector come together with road and education sectors in each country to address the problem. "A single intervention is not going to be enough," he said. "Education is such a huge contributor to lowering adolescent disease burden."

Saxena believes it is important to engage with children as they transition into adulthood.

"Adolescents are very vulnerable," she said. "That transition needs support ... and when adolescents don't have that support, you get risky behavior."

Ross believes the value this will bring to a population is underappreciated.

"Adolescents are an incredible resource for society," he said. "They need help and nurturing to be that."