The human body is basically a biological machine, and if it gets broken badly enough - by disease or physical injury - then sometimes it can't be repaired. And that's when we die.
Even if you're lucky enough to avoid serious illness or injury during your life, age will get you eventually. The cells in your body are frequently replaced during the course of your life; ageing occurs because of a process called 'senescence', in which the cells lose their ability to divide and make healthy new copies of themselves. It happens because of gradual damage to your DNA that accumulates throughout life.
DNA is the blueprint from which new cells are made, so as this masterplan is degraded, the quality of your cells also slips into decline. One leading theory puts the cause of the DNA damage down to chemicals called 'free radicals'. There are atoms and molecules that react strongly with other chemicals - and it's this reactivity that's eating away at your DNA.
The body has evolved defences, in the form of enzymes that repair the damage, but only 99.9% of broken DNA gets patched up this way. The tiny fraction that isn't stacks up over the course of your life, causing your body gradually to deteriorate: a process that leads to ageing and ultimately death. For a long and healthy life, perhaps the best tip is to limit your intake of free radicals - which means cutting out processed foods, cigarettes and excessive alcohol consumption. Boosting your intake of antioxidants (chemicals that block the action of free radicals) can also help - these are found in fresh fruit and vegetables, and green tea.
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