What is paresthesia (pins & needles) and why does it happen?

We all occasionally get “pins and needles”, the feeling that your skin is tingling or numb.  Typically, it’s caused by pressure on a nerve and is often temporary and harmless, like if you fall asleep on your arm or sit with your legs crossed for too long.  But there are many medical conditions, some more dangerous than others, that can trigger paresthesia, namely diabetes, carpal tunnel syndrome, multiple sclerosis, shingles, a herniated disk, hardening of the arteries, and toxins.