In the first 6.5 years of Operation Iraqi Freedom (OIF), U.S. military casualties exceeded 3,400 hostile deaths, 800 nonhostile deaths (due to disease, nonbattle injury, and other causes), and over 31,000 troops wounded in action. Casualty rates in Iraq have been considerably lower that during the Vietnam conflict, and a greater proportion of troops wounded in Iraq survive their wounds. Before the surge in troop levels that began in early 2007, the survival rate was 90.4% in Iraq as compared to 86.5% in Vietnam. Wounded-in-action rates increased during the first few months of the surge, but declined below presurge levels after the number of U.S. brigades in Iraq climbed from 15 to its maximum level of 20. Wounds during the surge were somewhat more lethal than previously, but because there were fewer wounding incidents the net effect was a reduction in the hostile death rate.