Short answers, pulled from the story.
Intellectual disability is a generalized neurodevelopmental disorder marked by significant impairment in intellectual and adaptive functioning that first appears during childhood. Diagnosis requires three things: significant limitation in general mental abilities, significant limitation in adaptive behavior across multiple environments, and onset during childhood or adolescence.
People with intellectual disability generally have an IQ below 70, defined as a performance level two standard deviations below the median for the test-taker's age group. The diagnosis is not based on IQ alone, since clinical discretion can apply to a person with a higher IQ but severe impairment in adaptive functioning.
Intellectual disability ranges across mild, moderate, severe, and profound levels. Mild ID covers an IQ of 50 to 69 and affects about 85 percent of people with the condition, moderate ID covers 35 to 49 and about 10 percent, severe ID covers 20 to 34 and 3.5 percent, and profound ID is an IQ of 19 or below and 1.5 percent.
Down syndrome is the most common genetic cause, followed by DiGeorge syndrome and fetal alcohol spectrum disorders. Other causes include infections like rubella during pregnancy, oxygen deprivation at birth, lead or mercury exposure, and iodine deficiency, which is the leading preventable cause where it is endemic. For one-third to one-half of cases in children, the cause is unknown.
Intellectual disability affects about 2 to 3 percent of the general population, and between 75 and 90 percent of those affected have the mild form. Cases of unknown cause alone affect about 95 million people, and the condition is more common in males and in low to middle income countries.
The term mental retardation became offensive over time through what is called the euphemism treadmill, the same process that earlier turned imbecile, idiot, feeble-minded, and moron into insults. The DSM-5 adopted intellectual disability in 2013 and the ICD-11 chose disorders of intellectual development, though mental retardation still appears in some aid and insurance paperwork.
Studies from the United Kingdom show people there with intellectual disability live on average 16 years less than the general population. Barriers to quality healthcare include communication challenges, service eligibility, lack of provider training, and diagnostic overshadowing.