War and occupation directly and indirectly claimed the lives of about a half-million Iraqis from 2003 to 2011, according to a groundbreaking survey of 1,960 Iraqi households. The violence peaked in 2006 and 2007, say public health experts who were part of the study.

On March 19, 2003, a U.S.-led coalition invaded Iraq, beginning a ground war that culminated in the rapid capture of Baghdad and overthrow of the regime led by Saddam Hussein. A coalition-led occupation of Iraq lasted until 2011, marked by repeated bombings, an al Qaeda-linked insurgency, militia warfare, and other bloodshed in the nation of 32.6 million people.

In the new PLOS Medicine journal survey, led by public health expert Amy Hagopian of the University of Washington in Seattle, an international research team polled heads of households and siblings across Iraq. The researchers, including some from the Iraqi Ministry of Health, aimed to update and improve past estimates of the human costs of the war and occupation.

"We think it is roughly around half a million people dead. And that is likely a low estimate," says Hagopian. "People need to know the cost in human lives of the decision to go to war."

The survey responses point to around 405,000 deaths attributable to the war and occupation in Iraq from 2003 to 2011. At least another 56,000 deaths should be added to that total from households forced to flee Iraq, the study authors estimate. More than 60 percent of the excess deaths of men, women, and children reported from 2003 to 2011 were the direct result of shootings, bombings, airstrikes, or other violence, according to the study. The rest came indirectly, from stress-related heart attacks or ruined sanitation and hospitals.

"Wars kill people all kinds of ways, not just in shootings. And it exacts a toll on the invaders as well as the invaded," Hagopian says. Some 4,804 U.S., British, and other coalition armed service members died in the invasion and occupation of Iraq.

Past estimates of Iraqis killed in the war and occupation have varied widely. U.S. Army war logs released by Wikileaks in 2010 pointed to more than 100,000, while a widely criticized study conducted by Opinion Research Business, a London-based polling agency, estimated Iraq war deaths at 1.2 million people through 2007.

"We had all Iraqis knocking on doors to ask the questions of these households," Hagopian says, explaining a 98 percent response rate reported from the survey. Heads of households were asked about family deaths, and household members were asked about sibling deaths stretching back decades.

"This is a really serious and credible piece of work," says epidemiologist Leslie Roberts of Columbia University in New York, who has led wartime mortality surveys in the Democratic Republic of the Congo, Zimbabwe, and Iraq. "I think having an accurate record of what happened is extremely important," he says, pointing to a 2005 comment by then U.S. President George Bush suggesting that only about 30,000 Iraqi civilians had died in the conflict.

Roberts agreed with Hagopian that the household survey estimate is likely conservative, because it relied on the imperfect recollections of household members and largely missed the 1.1 million Iraqis living in displaced-person camps or in other countries.

Overall, the survey results point to Baghdad as the epicenter of violent deaths during the war. Coalition forces were blamed for 35 percent of the killings, followed by militias at 32 percent. The report showed that warfare was particularly intense in 2007, followed by a sharp drop in 2008.

Sadly, the violence continues, notes Salman Rawaf, director of the World Health Organization Collaborating Centre for Public Health Education and Training, in a written commentary accompanying the survey. About 5,000 Iraqis have died in bombings and shootings this year, according to estimates by the French press agency, AFP. The return of sectarian violence means "living in Iraq today is no longer about how many have died, but how future deaths should be prevented," says Rawaf.