Lycanthropy is a curse (or a blessing, depending on where you stand) that can affect humanoid creatures, which forces the afflicted to transform into supernaturally powerful beasts. The curse appears to have some relationship with the Moons of Eberron, though this appears to differ between individuals. The origin of the condition is unknown, and there are some inconsistencies in how it presents itself that might point to there being multiple distinct variations of the condition, some of which are discussed in this article.
Olarune's Blessing
It is a common belief that shifters are descended from lycanthropes, but this belief is largely opposed by the shifter communities in the Eldeen Reaches, who believe that lycanthropy is a twisted form of their own natural gifts. The shifter's of the Reaches believe that their gifts are tied to Olarune's Blessing, a condition that spontaneously manifests (most commonly amongst the shifters in the Towering Wood) that grants the blessed the ability to transform into an animal, or take on that animals attributes in their humanoid form. The shifters believe this blessing is a sign, that they are being called to action by Olarune to protect innocents from the wild, and to protect the wild from those who would exploit it.
Those who are granted Olarune's Blessing find themselves drawn to embody the positive aspects of the animal form they've been granted, but it's not supernaturally imposed on them to do so. The Blessed can follow their own paths, but they will always feel the pull to do good and project the positive traits they believe the animal to have. Those carrying Olarune's blessing are only influenced by the moon Olarune, and feel a drive to become one with their animal form when the moon is full.
Olarune's Blessing cannot be passed on to others via a bite, it is non-transmissable. Those born to parents that carry the blessing may manifest it themselves, but this is not a given.
Dyrrn's Corruption
The Daelkyr Dyrrn was active in the Eldeen Reaches during the Xoriat Incursion that ultimately led to the fall of the Dhakaani Empire. The daelkyr was met by Olarune's Blessed, who had answered the call to defend their homes from this alien invader, and Dyrrn did what comes naturally to the daelkyr; it changed things.
Unlike with the goblinoids, Dyrrn didn't twist the form of the Blessed. Instead, it twisted the nature of the blessing itself, corrupting it and turning it into an affliction. Dyrrn's corruption is both hereditary and infectious, those affected by it can pass the infection on to others, but only if they themselves were born with it. Dyrrn's corruption is insidious, and it destroys the personality and most of the memories of those that become infected, compelling them to act on instincts that are instilled by the corruption.
Each strain of Dyrrn's Corruption has an associated set of behaviours, an associated moon, and a particular animal form. There isn't necessarily only one combination of these for each animal form, and so it's possible to encounter two individuals who transform into the same animal, but have a different associated moon and set of compulsive behaviours. The corrupted are only influenced by the moon that is associated with their strain of the corruption. The motives of the daelkyr are inscrutable, and the behaviours that the corruption compels in the corrupted are not necessarily predatory or threatening, as seen in some corrupted werebear's who feel a need to protect those weaker than themselves (which is almost everyone.)
The Curse of the Wild Heart
The Wild Heart is an Overlord from the age of demons, who embodies mortal fear of the wild and predation. The Curse of the Wild Heart is a strain of lycanthropy that is both hereditary and infectious, and it alters the behaviour of those who are afflicted and drives them to become predators. The curse is the form of lycanthropy that will force a werewolf to kill their own family, overcome with bloodlust. Unlike Olarune's Blessing, the afflicted do not embody any of the actual traits that are associated with their animal form, instead, they are driven to embody the fears and superstitions that are associated with them.
Those afflicted by the Curse of the Wild Heart are affected by all of Eberron's moons, and are forced into a transformation whenever any moon is full. The behaviour of the afflicted is extreme and violent, the curse is the source of the stories of lycanthropes slaughtering communities, and it was this that led to the Silver Crusade.
Natural born lycanthropes, those who have inherited it from their parents, feel the drive to prey on others but it isn't supernaturally imposed upon them like it is the afflicted. They can learn to control these impulses, and need not become the monster that many people fear they are. One notable example of this is the werewolf Zaeurl, the leader of the lycanthropes in Droaam, who is a natural born inheritor of the curse. She is a born predator, and she can be utterly terrifying when she wants to be, but she's not controlled by the curse and chooses her own path (most of the time.)
The Wild Heart's Curse fluctuates in intensity based on the current status of the Wild Heart itself. When the overlord is dormant, the curse can only be passed on by natural lycanthropes, afflicted victims cannot afflict others. When the overlord is stirring, or when individuals are in close proximity to it's prison, the curse is more powerful and any lycanthrope can spread the condition. The effect on the afflicted's behaviour is similarly effected by the Wild Heart's status, and when it is stirring the drive towards predatory behaviour is amplified and even natural lycanthropes will struggle to resist it.
Furthermore, the Wild Heart holds dominion over all forms of lycanthropy. During the Silver Crusade, the Wild Heart was close to breaking it's bonds and it dominated all of Eberron's lycanthropes, regardless of origin. Even the blessed of Olarune became cruel predators and hunted their own communities, contributing to the reputation that all lycanthropes are monsters. Those that survived the purge returned to their previous state when the Wild Heart returned to it's slumber, but the damage was done and the threat of a repeat looms.