Five to ten percent of the population suffers from dyslexia to some extent. Dyslexia is genetic and is significantly more common in boys than girls. The cause lies in the brain.
The two halves of the brain are constantly exchanging information. The right half is primarily tasked with processing information that enters via the eyes. The left half ensures that the information that has been received is systematically, automatically and quickly applied. This process of collaboration and information exchange between both sides of the brain is disrupted in people with dyslexia.
In practice, this results in persistent problems with reading fluently (word identification) and/or spelling (word visualisation). The problem can be worse when a second language is being learnt.