Milia, singular being milium, is from the Latin millet seed.
The cause is felt to be irritation which then causes the follicular oriface (pore) to close up. With the pore being plugged, the keratin produced in the follicle can not escape and forms the white bump you see. Milium are basically samll epidermal ( or keratinaceous cysts)...sometimes called sebaceous cysts, which is a terrible misnomer.
As I mentioned they are caused by irritation. Some of the things which irritate the follciular opening are makeup, moisturizers, sunscreens; in fact, anything which you put on your face. Thus, if you are noticing milia, you should switch skin care products, even if they are labelled as non-comedogenic.
These pesky little keratin-filled cysts can also be caused by the sun, so sun avoidance might be indicated. Any injury or trauma to the skin may result in milium.
They are so common in infants that they are dismissed as "milk bumps", though they have nothing to do with milk.
Milia are quite common after any ablative procedure or surgery. As the skin heals, there can be minute scar formation over the pore, the pore is plugged and keratin fills. They are frequently seen after dermabrasion.
A number of diseases cause milia such as porphyria cutanea tarda, epidermolysis bullosa acquisita, colloid milium, bullous pemphigoid. ( Rather than go into what these diseases are, if you have questions, e mail me).
We discussed milia in our last show June 18th) on www.blogtalkradio.com. If anybody knows someone who is interested in the modelling profession, we also hosted a nationally prominent advisor to models, Aaron Marcus. He was very enlightening and entertaining.