After destroying the samples, you'll receive orders to eliminate Dr. William Birkin, who is attempting to escape with the G-Virus. Follow the objective marker to a large research chamber. Birkin will be present, but he is not yet in his mutated form. He will attempt to flee, and you'll need to pursue him through several corridors.
During the chase, you'll encounter more USG and a few advanced infected, such as Hunters. Hunters are dangerous; they can leap great distances and deal heavy melee damage. Aim for their heads or use explosives to stun them. Eventually, you'll corner Birkin in a small lab. A cutscene will trigger, showing him injecting himself with the G-Virus. This marks the beginning of the first boss fight. Refer to the 'Boss Guides' section for detailed strategies on defeating G-Birkin Stage 1.