Why does everyone you know suddenly have ADHD?

Something weird happened in the last 12 months or so - it's the fact that all your friends - and perhaps you as well - have ADHD now. So where did this ADHD spike come from? Just how big is the gap between people seeking treatment and those able to give it? And if people are being overdiagnosed, or underdiagnosed, or diagnosed with the wrong thing... what are the risks?