The Parasitic Idea.
It starts out as an amorphous idea, that could grow and develop into anything. It lurks at the back of peoples minds in this state, a sort of hibernation. Anyone could be holding one in their heads and they wouldn't even know about it. It feeds off of idle thoughts and requires them to grow, but cannot directly partake of dreams.
Once the subject has a particularly lucid dream it moves into an active state. It begins to influence the subjects dreams every night to dream about it. The more the subject dreams, or the more vivid the subjects dreams the more detailed it becomes. As the SCP becomes more detailed its influence increases, eventually it begins to hijack daydreams, and will eventually take on a definitive form.
Once its form is defined, even vaugely it will continue to hijack the persons daydreams, though at this stage it is still too weak to take control of anything further by itself. This is where its secondary behavior comes into action. The SCP spreads from person to person via eye contact. Whenever the subject makes eye contact with another individual the SCP will spread, hijacking the daydreams and idle thoughts of two individuals. This affords it a measure of influence and control it would not have with a single individual.
Two however are not enough, and even as an amorphous half formed idea it still can spread in this manner. Eventually it has enough detail and power from a number of people, that it can actively sieze thoughts closer to the conscious and waking mind. Eventually the subjects will be unable to think of anything else and it is at this stage that it in a moment of irony, unthinkingly reveals itself. If it is not caught at this stage it becomes orders of magnitude more dangerous if it moves to the next stage in its life cycle.
Once it reaches the next stage the SCP will reach back through the barrier seperating the unconscious from the conscious and begin to actively influence and direct the thoughts and actions of the people under its influence. These people will move to actively spread the idea and defend it from physical threats.
The final stage in its life cycle has the SCP split into numerous amorphous ideas. The more people under the control and influence of the original ideas, the more new ideas that will form. One for each person. These ideas, however will have the control and influence of the person in whose mind they reside. They then spread to other population centers to begin the cycle once more.
The simplest and easiest way to attack these ideas is to simply shoot the person they feed off of. Once the mind is destroyed the idea will fade.