- ""...This virus only attacks those who have the gene that makes Telepathy happen. Normals like us will feel nothing. Those affected will need injections daily, or they will suffer an agonizing and terrible, painful death...""
- – Edgars about the Telepath Virus
The telepath gene exists in many sentient races. Those with the gene are called telepaths and have the ability to mentally probe others and hear or see what they are thinking, have experienced or felt.
The Narn had viable "weapons grade" telepaths as early as the First Shadow War, though that population was wiped out after driving the Shadows from the Narn Homeworld. Narn telepaths were known as "the Mind Walkers." 
While the Psi Corps was in existence, all humans were tested at a young age by regular blood screenings. Anyone with a positive marker is taken in for training and testing. The gene was strengthened by genetic coupling in the Psi Corps, which helped increase the chances of a P10 or higher.