Betazoid

Physiology

Externally, Betazoids were physically indistinguishable from Humans in every aspect but one: the irises of theireyes were completely black. They could even cross-breedwith Humans, along with other humanoid races likeKlingons and Tavnians. Betazoid iris coloration is present in half-Betazoid individuals such as Deanna Troi, but in those with less Betazoid blood such as Devinoni Ral (who is only 1/4 Betazoid), normal human eye coloration is possible.

Betazoids reach rapid eye movement (REM) sleep at a different frequency from other humanoids.

In mid-life, Betazoid females undergo a physiological change known as "The Phase" during which their sex drive increased by a factor of four or more. 

Mature Betazoids could also suffer from Zanthi fever, which caused them to lose control over the projective aspect of their empathy. That could result in people around them acting as if they had experienced the same general emotional state as the affected Betazoid, though they were directed by their own subconscious desires, fears, and other emotions.

Telepathic Abilities

Betazoids were natural telepaths, an ability centered in their paracortex, with psilosynine being a mainneurotransmitter.

Most developed their telepathic skill in adolescence, but a few were born with their telepathic abilities already active. These individuals were almost always extremely talented and powerful in telepathic terms, but also unable to screen out the noise of other people's minds, so they generally suffered mental problems of varying severity depending mostly on when the problem was diagnosed. On the other end of the scale were a few individuals who develop psionic abilities which were far below average for this species

The common psionic abilities of Betazoids extended from sensing thoughts and/or emotions, over projecting thoughts and/or emotions, to manipulating the minds of others.

How capable they were in performing any of those feats depends somewhat on their genetically defined psionic strength, their psionic training, their familiarity with the scanned being, their general mental and physical condition and the species of the subject race. Betazoids had been able to read the emotions/thoughts of non-corporeal beings such as Nagilum, the energy vortex, or gauge the mental prowess of beings such as Q. Other times they had been totally unable to read corporeal creature's minds, such as Ferengi and Changelings.

Inter-species reproduction involving Betazoids often affected the psionic abilities of the offspring – most commonly the children of such a union develop empathic abilities as their primary psionic talent, while their telepathicabilities, though existing, were rather below average for Betazoids. Usually the telepathy of these half-breeds, without extensive training, was limited to communication with other empaths or telepaths and full telepathic contact with emotionally very close persons (for example, an imzadi). All full Betazoids are unable to read the thoughts of Ferengi, Breen, Ullians, or Dopterians, but some half-Betazoids have been occasionally successful in sensing the emotions of some of these species. Additionally, even full Betazoids seemed to be able to psionically influence some of these species.

Illness and Diseases know to species:

Zanthi fever
Virus which affects the empathic abilities of Betazoids. Sufferers project their emotions onto others.