I would not focus on individual cells but the ability to grow super fast, which itself is not a defect since it's important during development, but the checks on this ability can be removed by later mutations/metabolic issues

The problem is using cancer as a singular noun in this context.

>a cancer grows from a cell that a body needs in small, healthy amounts

"A" cancer does not grow from a cell, a cell is a cancer cell if it keeps dividing when it should not. The cancer cells as a collective are the disease referred to as "cancer".

If they had written

"cancer grows from cells that a body needs in small, healthy amounts, and that turns deadly when it grows without limit until it harms, or even kills, its host"

that would have made more sense to me.