Cancer is a minimum effort evil for the spiritual powers of evil. They do need to concentrate extensively. All they need is to corrupt a couple of cells somewhere in a human body. They distort the cells, so that they begin to divide and grow rapidly.
For a long time, the cells are too small to be noticed, but they are growing away like a ticking time bomb. When the time is right, the disrupted cells becomes a growth so large that it causes the body carrying it to become sick. If left alone, it will becomes so large and spread so far that the person afflicted will die.
The spiritual powers of evil have the skill to be able to do this, because they were around at the creation before their fall. They helped God create the plants and the animals, so they understood the structure and function of cells, better than we do.
After the fall, they had plenty of practice corrupting cells and species, during the millennium of darkness. Causing a couple of cells to divide and grow rapidly is child's play for them.
The spiritual powers of evil have their favourite places to work. They like to work with reproductive organs, because they love to destroy life. Their favourite place is the bowel, because they are comfortable working amongst the excrement of life.
Starting a cancer takes minimum effort, because once a couple of evil spirits have corrupted the cells and set them growing and dividing, they do not need to do anything. They have created a time bomb that will grow and explode without further effort.
All they have to do is send a spirit of fear when the cancer is discovered and the job is done. A human life is destroyed.