In infants up to the age of 2, according to the American Academy of Pediatrics, regular TV exposure results in poor brain development due to the TV's inability to react to their behavior.
Dr. T. Berry Brazelton and Dr. Matthew Dumont (Harvard Medical School) have found that TV is a primary factor (but not the only one) of ADHD in pre-adolescents. That finding sounds extremely counterintuitive. As a former hyperactive child (and still hyperactive and easily-bored), though not one who was ever diagnosed, I believe that truly hyperactive children are too impatient to spend nearly as much time watching as much television as "normal" children do. -- DanielKnapp
Johns Hopkins, the CDC, and the NIH have determined that TV is the primary cause of childhood obesity, which itself is the primary factor in adult obesity. The diseases that result from such obesity are myriad, including such killers as diabetes and hypertension.
More on this at http://www.sover.net/~gmws/untv/develop.htm ...
Also see PhysicalEffectsOfTelevision?, TeleTubbies, and JustPlainLazy?
What about PhysicalEffectsOfHacking??