Dr. Weber gave you this syringe filled with some of the chemicals that create the Infected. He requested that you use on any person who might seem like an interesting candidate to study. The syringe reminds you to avoid accidentally sticking yourself with it, along with an experience you call...
It all started when you met Dr. Weber in Mercy Island. Weber informed you that he was researching those people who had become Infected and how to leverage the apparent abilities and sickness that it gave them. He asked you to infect several members of the RIP with the virus to see what happened to them. Sure enough, as soon as the chemical was injected into their bodies, they transformed into Infected. However, something was wrong; someone or something was calling out to the Infected, trying to get them to go somewhere.
This news disturbed Weber, who had plans to try to control those who had been infected, with the hopes that you would be the villain who would take charge of them. Weber located a number of these mind-controlled beings and asked you to see what was happening with them. After not-so-politely dealing with the sickened people, you found a gathering of them outside the walls of Mercy. It appeared there was some sort of being who was seen as a leader of the Infected, and he too, was heeding the call, apparently being made by Longbow.
You took down several members of Longbow and discovered a base they had set up, where a heroine by the name of Psi-Lass was using her psychic abilities to call the Infected. Longbow believed that they had a cure for the Infected and were intent on gathering them all together to deliver it. You put a stop to Psi-Lass and Longbow's research into the cure, ensuring that no person would try to stop the Infected from growing.
Dr. Weber informed you that with your help, he would be able to properly continue his research into controlling the Infected and would be able to use them to help you stop Longbow. One thing stuck in your mind from your dealings with Weber. He never saw himself as a villain, as taking advantage of these people. On the contrary, he seemed to believe he was doing the people of Mercy a favor, giving them a higher purpose than whatever role they were currently playing in their life.