A common vampire bat begins to expel urine within two minutes of feeding. While shedding much of the blood's liquid facilitates flight takeoff, the bat still has added almost 20–30% of its body weight in blood. To take off from the ground, the bat generates extra lift by crouching and flinging itself into the air. Typically, within two hours of setting out in search of food, the common vampire bat returns to its roost and settles down to spend the rest of the night digesting its meal. Digestion is aided by their microbiome and their genome protects them against pathogens in the blood. Although rare, infection of humans by rabies from vampire bat bites has been documented; for example in 2010 four children in Peru died after being bitten. The highest occurrence of rabies in vampire bats occurs in the large populations found in South America. However, the risk of infection to the human population is less than to livestock exposed to bat bites. Only 0.5% of bats carry rabies, and those that do may be clumsy, disoriented, and unable to fly. The unique properties of the vampire bats' saliva have found some positive use in medicine. A study in the January 10, 2003, issue of "Stroke: Journal of the American Heart Association" tested a genetically engineered drug called desmoteplase, which uses the anticoagulant properties of the saliva of "Desmodus rotundus", and was shown to increase blood flow in stroke patients.