Pneumonia claimed the life of a child under five every 39 seconds last year, according to a new report.
Save the Children said the disease killed more than 800,000 children in this age group in 2018 and that it was a "forgotten global epidemic".
Most deaths occurred under the age of two, and almost 153,000 within the first month of life, said UNICEF.
Pneumonia is caused by bacteria, virus or fungi. The first symptoms are a heavy cough, diarrhoea, vomiting and serious breathing problems, as the lungs are filled with pus and fluid.
More children under the age of five died from the disease in 2018 than from any other: 437,000 children under five died due to diarrhoea and 272,000 to malaria.
However, this infection is preventable and can be cured.
“Every day, nearly 2,200 children under the age of five die from pneumonia, a curable and mostly preventable disease. Only through cost-effective protective, preventative and treatment interventions delivered to where children are will we be able to truly save millions of lives," said Henrietta Fore, executive director of UNICEF.
The disease can be prevented with vaccines, and easily treated with low-cost antibiotics if it is properly diagnosed. Children with severe cases of pneumonia may also require oxygen treatment, which is rarely available in the poorest countries to the children who need it.
The most vulnerable are those who live in poor conditions, in areas with high levels of air pollution or that suffer from malnutrition or HIV.
“This is a forgotten global epidemic that demands an urgent international response. Millions of children are dying for want of vaccines, affordable antibiotics, and routine oxygen treatment. The pneumonia crisis is a symptom of neglect and indefensible inequalities in access to health care", UK Chief Executive of Save the Children Kevin Watkins said.
In January, world leaders will meet in Spain at the Global Forum on Childhood Pneumonia to take global action.