A learning deficit can have many causes, such as vision or hearing problems, limited intelligence, or simply raising the child in an environment with very few social and cultural opportunities.
Dyslexic children though have normal or superior intelligence and good health generally. There does not seem to be any particular reason that causes dyslexia.
Many theories have been formulated in an attempt to explain the symptoms of dyslexia. It may be due to a dysfunction in specific regions of the brain, which is otherwise normal. However, nothing has yet been proven.
However, children with dyslexia often have close relatives who are dyslexic, which indicates a hereditary link.