What can cause optic neuritis in a healthy 17 year old girl?

Mostly demyelinating. There are multiple possible causes. While almost always demyelinating, other causes include lyme's disease, sle, sarcoidosis, and pseudotumor cerebrii. There are an number of causes for demyelination including post-infectious (adem) and multiple sclerosis. There are also some nutritional causes. The best way to determine the cause in a healthy young woman is to see a neuro-ophthalmologist.
Optic neuritis. The most common cause of optic neuritis in an otherwise healthy individual is multiple sclerosis. An examination by a neuro-ophthalmologist and neurologist are recommended to evaluate the possible cause for the optic neuritis.