We are constantly under attack. Chemicals in the environment, ultraviolet light, even cosmic radiation — our DNA is bombarded 24/7 by agents that can cause damage and mutations. But don't take my word for it.
"It is estimated that a single cell in mammals can encounter approximately 100,000 DNA-damaging events per day," says Emi Nishimura of Tokyo Medical and Dental University, who works on the biology of aging. Given that there are something like 10 trillion cells in the human body, that's a battle on an epic scale.
Most of the DNA damage is unavoidable and fixed — being the reason, for example, why our skin wrinkles and we start to look old. But something can easily go wrong.