Sitting in front of a computer now poses a risk of developing DVT (Deep Vein Thrombosis), where blood clots, blocking blood flow, and causing tissue damage, organ damage, brain damage, or even can prove fatal.

The first person to develop e-thrombosis sat at a computer for up to 18 hours a day.
A blood clot developed in a vein in his leg, broke off and travelled up into his lungs.

It is advised that people who use a computer for long periods of time need to take frequent breaks, they should undertake frequent foot and leg exercises, and should get away from the screen

http://news.bbc.co.uk/1/hi/health/2698119.stm