A cell is a term for the building blocks of life. All living creatures are made up of cells. If you had a powerful microscope, you'd be able to look at your hair, or skin, or whatever and see them. Your DNA is in your cells, which is what makes you 'you', and what parents pass on to their offspring. Cells reproduce via division. For example, when you cut yourself and you 'grow' new skin to close the wound, that's your cells dividing. Cancer is when they don't stop growing, so you end up with extra bits and pieces of things where they shouldn't be.
KA happened because some morons broke into a medical facility and released a virus that wasn't thoroughly tested yet. Two viruses met, they mutated, and KA happened. KA was not the intended result.