After completing the House of Science quest, talk to Dr. West to begin this one. You must find the natives. When I played this mission the mini-map selected a path that didn't work. Get in the truck and head on down the road south of the Laboratory and the mini-map should re-route a better way.
When you reach the entrance you will have to find another route in. Take the path just to the left of the entrance. You will come to a gladiator style fighting ring where you will have to fight a bunch of infected. Many of them have a fart cloud around them so any weapon with the Torch Mod will be very helpful. After the infected you will have to fight an infected warrior as well as some more infected. This can be tough so use your fury if you have it. Once you defeat the warrior, take the blood sample back to Dr. West. This will trigger the Demonic Science quest.