First, let's define "half-blood". Being a half-blood, or "demigod", means that you are half moral, half god. There are gods of all sorts: Greek, Roman, Nordic, etc. Here at Camp Half-Blood, the demigods are children of Greek gods and goddesses.
To the mortal world, demigods don't appear to be any one special. They look like normal kids. The difference is, however, these kids have a learning disability as well as medical condition.
The learning disability that these kids have is dyslexia. What this means is that these kids have trouble decoding and processing words, making reading comprehension difficult for them.
There are three types of dyslexia:
For Half-bloods, being dyslexic is a struggle in the mortal world. However, the reason they are dyslexic is because their brains are hardwired to read Greek. Being half god means that they struggle to read and understand english. Mortal teachers and school administrators see this as a "flaw" and identify these students as dyslexic.
The medical condition that demigods have is Attention-Deficit Hyperactivity Disorder (ADHD).
ADHD is described as a disorder that causes the student to become either inattentive, disruptive, or irrational inside and outside of the classroom. There are six different sub categories of ADHD:
Boys are most commonly diagnosed with ADHD-H and ADHD-I while girls are often diagnosed with ADD/I and ADD/WO.
Half-bloods struggle to sit still in classrooms and often make impulsive decisions. Again, because they are half god, demigods are born with a sense of fight within them. AHDH allows the half-bloods to be hypersensitive to their surroundings in battle and to instinctively sense their opponents next move.