Infectious illnesses such as pneumonia, diarrhea and malaria accounted for more than two-thirds of the estimated 8.8 million deaths of children under five worldwide in 2008, according to a new study.

The study, published in this week's online issue of the medical journal The Lancet, was done by researchers at Johns Hopkins Bloomberg School of Public Health in Baltimore for the World Health Organization and the United Nations Children's Fund (UNICEF).

Of the 8.795 million child deaths in 2008, 68 per cent, or 5.970 million, were caused by infectious illness, Robert Black and his colleagues reported.

Pneumonia (18 per cent), diarrhea (15 per cent) and malaria (eight per cent) were the biggest killers.

Almost half of the child deaths worldwide occurred in only five countries: China, Nigeria, India, the Democratic Republic of the Congo and Pakistan.

At least 41 per cent (3.6 million) of the deaths occurred in the first 27 days after birth.

"These country-specific estimates of the major causes of child deaths should help to focus national programs and donor assistance," the study's authors concluded.

The number of children under five in the world has increased since 2000, but the number that die each year has fallen, the study's authors noted. Between 2000 and 2003, about 10.6 million children under five died each year — compared to 8.8 million in 2008.

One of the UN's eight millennium development goals set in 2000 is to reduce child mortality by two-thirds by the year 2015.

"[That] is only possible if the high numbers of deaths are addressed by maternal, newborn, and child health interventions," the study said.

The study was funded by the WHO, UNICEF and Bill & Melinda Gates Foundation.