The military secretly has a unknown drug that releases a chemical into the brain which sends you on a trip on which your vision is changed so that you now see in "night vision" seeing heat signatures instead of detailed images. The drug proves successful with the military in the night "spec ops" missions taking the need for the bulky night vision goggles out of the equation. The soldiers are faster and more agile in the darkness. There is one downfall with the drug though. It is extremely addictive and after long enough continuous use some of the soldiers were at the point of feigning for the drug. It got bad enough to the point where the military had to move the drug to a secure location on the base stored with other unknown chemicals. it didn't take long for the feigning soldiers to track down the drug and attempt to take it all for themselves. During the heist though the drug was spilled on the ground along with another unknown chemical. The addicted soldiers saw their valuable drug on the ground and disregard the other unknown chemical and start to lick the new mixture off of the ground. The chemical mixture spreads throughout the soldiers body changing them at a rapid pace. Their skin is becoming rotten and morbid looking. in a matter of minutes the soldiers look like a 3 month old corpse and suddenly they now have a desire for a different "drug", human flesh. Thus starting the zombie apocalypse. Thank you U.S. military.