Under normal circumstances, your body produces a steady flow of healthy blood cells. We'll keep it extremely high-level and say that you've got your white blood cells and your red blood cells. There are a few more than that, but all we're concerned about is ability to fight infection (white) and ability to clot (red).
It's your bone marrow, actually, that does the production. Through some process far beyond my understanding, healthy bone marrow produces a healthy string of different types of blood cells, as appropriate.
Somewhere along the line -- whether it's genetic, or environmental, or some combination of the two -- the bone marrow of a patient with leukemia will begin producing immature blood cells. They're not nearly as effective as the stronger, healthier cells. Certain subtypes of AML will tend to see certain symptoms, based on which production line gets thrown most off-kilter. All will share some common symptoms, mostly related to the growing number of white blood cells in the body that aren't able to do their jobs: fatigue, lingering sicknesses, a general feeling of malaise.
The subtype that I had (M3, now classified as APL, or Acute Promyelocytic Leukemia) is also generally recognized for a very low number of healthy platelets (a kind of red blood cell) which helps blood to clot around wounds. With these patients, you will see a great deal of bruising and bleeding.
Left unchecked, the bone marrow will continue to fill the blood stream with larger and larger quantities of malformed blood cells -- "blasts," to borrow a bit of medical terminology. There's probably some kind of accelerated curve, the greater the percentage of blasts, the more quickly they begin to crowd out the remaining few healthy blood cells.
Eventually (again, left unchecked) the patient will die from some infection that the body isn't able to respond to, or from some internal or external wound that simply doesn't stop bleeding. There's nothing left. You can't stop it. Your defenses are gone.
Leukemia is a passive-aggressive killer. It doesn't do anything by itself, other than undermine a body's ability to stave off something else. Deadly? Not me, Leukemia says. It was that other guy. The brain infection. Not my fault.
Sneaky, quiet, back-stabbing mother-effing disease.