A virus that is deadly to magical beings
The American government
When the government found a wounded Krychek Demon walking around in the desert, they somehow managed to capture it. As part of an experiment, they injected the demon with a human blood sample. Their goal was to retrieve the sample and inject it in soldiers to grant them demonic powers. However, the experiment instead created a virus that quickly spread across the magical community through exposure to the infected blood.
The infected Krychek Demon escaped and infected several magical beings, including Gnomes and Leprechauns. Eventually, it attacked Magic School, which was used as a base by the demon Margoyle and his demons. Once there, the infected demon attacked another Krychek Demon named Emrick and infected him. The Charmed Ones, who had been sent after the demon by Agent Murphy, saw this and fled when the demons attacked them.
On a quest to find her sister, Billie Jenkins infiltrated the school and came face-to-face with Emrick. When the virus took hold of Emrick, he attacked Billie at the Halliwell Manor and infected her before Paige orbed him back to Magic School. There, a Soothsayer experimented on him before he died.
When the virus took hold of Billie, she attacked Phoebe before Piper knocked her out with a blast. Piper then summoned the Elders and Jonnah revealed that the virus was spreading across the entire magical community.
The Charmed Ones confronted Agent Murphy and demanded answers. He revealed that only one test subject was immune to the blood, meaning that he held the cure. In order to find him, the sisters infected themselves with the virus and used the power boost to locate this Patient X. After fighting off Margoyle's gang of demons, they obtained the blood sample and cured Billie and themselves. The Elders then ensured the cure was spread across the magical community. However, Margoyle managed to obtain a blood sample as well and gave the cure to demons.
The Effects of the Virus
The Virus is spread through injuries and direct blood to blood contact. The effect of the virus is dependent on the power of the infected. The more powerful a being, the quicker the virus will kill them.
In the first stage of infection, the infected will undergo a physical transformation that will cause them to hulk out, gaining super strength and durability and making them highly aggressive. Their strength is heightened to the point that they can easily vanquish demons by knocking them over. Most of the infected will go into a frenzy and will be unable to reason, though the Charmed Ones were able to retain their senses.
In the second stage of the infection, the infected will return to their original form and will be left severely weakened as the virus courses through them and eventually kills them. Due to this weakened state, Piper Halliwell was able to freeze Billie Jenkins to slow the infection, despite the fact that good witches are immune to this power.