Rage virus

The rage virus is a bacteria, transmitted through contact with bodily fluids, that acts as a kind of mutagen, transforming the victim from the inside out.

Origin
The first signs of this rage virus appeared from the Gamma Quadrant, where a race of violent non-humanoids were breeded by the Dominion for a biological war against the First Federation. The alien blood would mutate their enemies if ever they got it in themselves. This plan went forward, but the consequences were shocking.

Appalled by these rage monsters, the First Federation united with all of its former enemies (the Klingon Empire, the Romulan Star Empire, the Cardassian Union, the Gorn Hegemony, the Tholian Assembly, among others now extinct) against the Dominion. The allies all disliked each other in their own way, but they all knew that, by themselves, they were weak against this greater threat.

The rage monsters were put down, until it was believed that they were all extinct.

Reemergence
The rage monsters may have died, but it was hardly the end. Their blood, which remained after death, infected all who touched it. They also became rage monsters and, unhindered by any kind of weaponry, went wild. This return of the rage monsters lasted for some time, until a wise-man named Landru used his healing powers to purge the patriarch rage monster. Many others followed, and were cured.

But the last female rage monster remained at large. Facing the extinction of her race, she learned, through rigorous self-inflicted torture and mutilation, the way to suppress the rage virus long enough to assume her original form. The virus continued to live inside her, guiding her actions with its thoughts, and in turn, she had longer lifespan than most others.

In 2154, she encountered Gradl Peschtal. She had "relations" with him, but the alien rage virus made her sterile. However, the virus passed to him. When he learned this, he killed her and then killed himself and had himself re-cloned in order to flush out the virus. It remained in his DNA for many years, and many other re-clonings, but it did not pass on to any of his nine children...yet.

Signs of Infection
One rarely sees outward results of the infection until it is too late. The first symptoms are slight mood alterations: for example, a timid, passive person might suddenly become aggressive or hostile. These "mood swings" continue, getting degressively worse and worse.

At this stage, the virus is almost untreatable. With the mood becoming under its control, the virus will begin to take control of the victim's physical body. The process is slow and painful, and lasts for many weeks. On rare occasions, the virus might take full control in a matter of seconds: this is extremely rare, and only happens if the mind is suddenly opened by an invading force.