The picture below shows the dangers of a concentration of risk in one asset.

In the past days the FTSE has moved little, with highs of over 7,200 to lows of 7135. That is about a movement of 1%

BT plc has traded this week from £3.80 to as low as £2.99. That is price movement of 20%

The benefits of a fund like a FTSE-100 tracker is obvious. Never have all your eggs in one basket.