We are made up of billions of tiny cells, and these are constantly dividing to make more cells, in order for us to grow, recover from injury, or even just as part of a natural turnover as old cells run out of energy and die to be replaced by new cells. Different cells belong in different places in the body and have different functions – as I learned in my junior cert, different groups of cells working together make up tissues, different tissues working together make up organs, organs working together make up a system, and systems working together make up a human!
Normally, our body senses when we need to make new cells, and as soon as enough new cells are made, we stop making them. Cancer happens when cells start to grow and reproduce uncontrollably. They grow too fast to form properly and too many of them are formed. Tumors form then, which are big collections of fastly growing cells, and they take away valuable resources, like food and oxygen, from our normal, healthy cells, and we start to suffer. Thankfully we now understand the differences between cancer cells and normal cells enough to be able to develop some really good treatments for cancer.