Therianthropy

The most famous example being the lycanthropes or "werewolves", the disease family known as therianthropy causes the inflicted to shift from humanoid form into an animal form under certain conditions. Most strains have two types of victims: those born with it and those who were infected later in life.
The "natural" therianthropes are able to control their shifting and could infect others. Many natural were-beings are feared as a direct result of werewolves and their violent habits.
The infected or inflicted therianthropes are typically forcibly shifted when a particular event happens, such as a full moon for a werewolf, but some versions do have similar levels of control to the natural ones (practice time being the only true barrier).

Transmission & Vectors

Blood and spit of someone born with the condition are the usual vectors, but magicians have long mastered replicating the process used to make one so curses manifesting as the disease are common enough.

Causes

The exact beginnings of the disease are debated. Some cite sources saying it was a curse put on a person who shamed the gods with just as many claiming it to be a gift. Generally, the idea of it having been both with a combination of other things is considered the truth.

Symptoms

An "allergy" to silver, healing so quickly from non-silvered non magical damage that it essentially never occurred, slight altering of personality in infected, and (of course) an ability to shift (completely and partly) into an animal without a spell.

Treatment

A creature born with the disease cannot be healed of it completely, but any wealthy or connected enough victim can have a mage create or find an amulet that cancels out the worst of the symptoms.

Type

Supernatural

Children

• Ravenkin

Origin

Magical

Cycle

Chronic, Acquired & Congenital

Rarity

Uncommon

Affected Species

• Humanoid
• Dusk Elf

Disease or Species?

It's best to note here that while this has classified therianthropy as a disease, reality is far more messy. In practice, the therianthropes are often treated as separate races by the rest of the world. A human that was turned into a werewolf may be able to pass as a normal human for a time, until they get spotted shifting in the moonlight and outed as a lycan. No matter what race the therianthrope's ancestors may have been, they will be defined as a shapeshifter, set apart from the rest of the humanoids. Depending on the type and current location, they may be treated as monsters,  as a friendly, but different, species, or somewhere between. For our purposes, however, it works best to classify them as diseased humanoids, even though the connotations makes me wince.