Every year 1.9 million children under 5 years of age die from pneumonia (1
). Indeed, it is the leading cause of child death in the world. Pneumonia is an acute illness in which the alveolar air spaces of the lung become inflamed and filled with fluid and white blood cells, giving rise to the appearance of consolidation on the chest radiograph. It can be caused by bacterial, viral, or parasitic infection as well as by noninfectious agents. Most severe cases of pneumonia are caused by bacteria, of which the most important are Streptococcus pneumoniae
(pneumococcus) and Haemophilus influenzae
. In developing countries, where patients are often treated without seeing a doctor, the WHO defines clinical pneumonia simply as an acute episode of cough or difficulty breathing associated with an increased respiratory rate (2
Pneumonia is a disease of all ages, and in adult medical wards across the developing world it is one of the most common admission diagnoses. In contrast to the industrialized world, pneumonia is found characteristically in younger adults, who have a substantial inpatient mortality of 5%–23% (3
). The pathogens causing pneumonia in children and adults are similar, and most respiratory pathogens are transmitted effectively between generations within households. In the United States, preventing pneumonia in children by vaccinating against pneumococcal disease has resulted in less pneumonia in adults (4
). However, little is known about adult pneumonia in developing countries, and research is rare outside the context of emerging infections (3
). There are thus considerable opportunities for pneumonia research on adults. However, in this Review, we concentrate on childhood pneumonia and specifically on research to reduce the unacceptable magnitude of child deaths from this disease.
Historically, pneumonia was the main cause of child death in developed countries, and in the United States in 1900, it is estimated that pneumonia killed 47 of every 1,000 children before the age of 5 years (5
). Improvements in nutrition and living standards in the United States in the first 40 years of the 20th century led to a substantial reduction in pneumonia mortality well before antibiotics became available as an effective treatment (Figure and refs. 6
). However, in the low-income countries of Asia and Africa, pneumonia is still the main cause of child death. In developing countries, over one-quarter of children have an episode of clinical pneumonia each year throughout the first 5 years of their lives (9
). On average, 2%–3% of children each year have pneumonia severe enough to require hospitalization, and many of these disease episodes are potentially fatal (9
). Thus, for every 1,000 children born, about 100–150 episodes of severe pneumonia arise during the first 5 years of life, most during the first 2 years. Approximately 21% of child deaths are due to pneumonia (1
), and many developing countries have mortality rates of 60–100 per 1,000 children under 5 years of age (10
); this suggests that of every 1,000 children born alive, 12–20 die from pneumonia before their fifth birthdays.
Mortality rate attributable to either pneumonia or influenza among children aged less than 5 years in the United States, 1900–1960.
Although we know the number of children who die of pneumonia in developing countries, few of the causal factors leading to death have been elucidated. Mortality is associated with poverty and with malnutrition; critically, we do not yet know the extent to which it is caused by lack of access to health care. If access to care is inversely related to mortality, the impact of new diagnostics, medical treatments, and vaccines will be substantially constrained. Therefore, the first priority for pneumonia research is a better understanding of the epidemiology of fatal pneumonia. Increased understanding of this, as well as more detailed information about the etiology and pathophysiology of the disease, should guide new approaches to tackle the immense global problem of child deaths from pneumonia.