# Chapter 9: When cells go crazy

The human body is a multicellular organism composed of cells whose number changes according to age, height, weight, and state of health, but also due to environmental factors. The average number of cells is said to be 37 trillion, and they can be divided into about 200 different types. Just like human beings, the cells of their bodies do not live forever. Most cell types can be replaced by new cells after their death, through cell division, which has already been discussed in Chapter 3 - Meet DNA, the bearer of genetic information. The division is a cyclical process without which life would not be possible. The problem arises when cell division gets out of control.