Level D or Ex-Humans are humans who have become vampires as a result of being bitten by a pureblood vampire. As opposed to all other classes in the pyramid who are born as vampires, ex-humans were originally human before being bitten and transformed into vampires.
There is a difference between stabilized and unstable Level D vampires. If an ex-human receives blood from the pureblood who turned into a vampire, them then their condition is stabilized. If they do not receive that blood, then their transformation remains unstable and ultimately unsustainable. They will eventually become eaten away by bloodlust and go mad, falling into “Level E” status. At that point the unfortunate person must be put down as there is at present nothing that can be done to save a Level E once they have fallen into madness.
Not all humans who are bitten by a pureblood become vampires, some react badly to the change, their blood becoming toxic, and they die instead.
It is currently against the law for a pureblood to turn a human against their will, although it still happens. Sometimes humans wish to be turned and request it. Since humans as a whole are not aware of vampire existence, it is to be assumed that for the most part those who request transformation are probably already in a relationship with a lower class vampire and discovered the truth in that manner.