Some diseases are called cancer. They all have one thing in common: in such a disease, cells in the body divide and multiply. Cells do this anyway, but in cancer they do it unchecked and without regard for the other cells.
Normally, it makes sense that cells can divide. If they did not, wounds, for example, could not heal. Every cell has a use in the body. In cancer, the cells lose their actual use. They only multiply. In the process, they not only destroy organs in the body, but can also spread throughout the body. There, in a completely different place, they continue to grow and destroy other organs.
This is why cancer is also called a "malignant" disease. The cancerous growth is called a "malignant tumour". There are also "benign tumours". They also grow, but do not destroy other organs and do not spread in the body.
Cells in the blood, the white blood cells, can also grow uncontrollably and lose their function. They flood the blood and prevent other blood cells from functioning. This is called leukaemia. Leukaemia can be detected in the blood under the microscope. In the case of tumours, cells are removed in a small operation, which the doctor then examines under the microscope.