Cancer

 Once cells feel too cramped they will have "contact inhibition", when they feel they feel something wrong about themselves, they kill themselves.

There are actually around 37.2 trillion cells in the human body.

Let's say there's a cell with a couple of mutations (there's a small chance every cell in the S phase can mutate, most of the time it's harmless, and sometimes it's very harmful).
Let's say there's a cell that has a mutation that stops it from experiencing Apoptosis which is what kills the cell, and a mutation that causes it to replicate faster, and this cell quickly makes lots of copies of itself during mitosis, and the large amounts of copies have defects, as they can't kill themselves if they feel something is wrong. This is called a neoplasm.

Most of the time they combine together and form a lump and then if it gets large enough it becomes a tumour. If it becomes a certain size it becomes a Benign tumor which is harmless. But let's say the Benign tumour has a mutation that causes it to grow fast and it might become invasive, where it wants to infiltrate everything and invade other tissues, but not yet, he's still a cell, and what do cells do?.
They Replicate, so there are lots of these cells running around trying to infiltrate everything around them causing chaos, maybe another mutation causes it to break off from the group and start attacking other parts of the body, organs, heart, and lungs.

Thus the cells have Metastasized, and they keep mutating, keep causing problems by taking other cells' jobs and hurting the body, they are called Cancer Cells.