cancer remains one of the most frightening diagnoses patients can receive.

What makes it so difficult to cure is that it's not one illness,

but a family of over 100 diseases occurring in different types of cells.

And one type of cancer has the unfortunate distinction

of afflicting children more than any other type.

This is leukemia,

a cancer that begins in stem cells found in the bone marrow.

A stem cell is a bit like an infant,

undeveloped but possessing great potential.

Many stem cells specialize and become cells of organs,

like the liver, brain and heart.

But in some tissues,

they can continue to divide into new stem cells throughout development,

and into adulthood in order to frequently generate new cells

and keep up with the body's needs.

One example is the bone marrow,

where stem cells differentiate into many types of blood cells.

That includes red blood cells,

which carry oxygen from the lungs to all tissues,

platelets, which help stop bleeding by sticking to damaged blood vessels,

and white blood cells,

which patrol the body, destroying potentially harmful invaders.

Every once in a while,

something goes wrong during a stem cell's specialization process

and harmful mutations occur in the cell's DNA.

Cells with compromised DNA are supposed to self-destruct,

but some damaged cells ignore this order,

replicating uncontrollably, even as they lose their original function.

These are what we know as cancer cells.

It is not yet clear why leukemia is the most common childhood cancer,

but one contributing factor may be

that leukemias are often caused by just one or two DNA modifications,

while most cancers require many of them,

allowing leukemias to arise faster than other types of cancer.

Moreover, some DNA alterations can occur in white blood cells

during fetal development,

further increasing the risk of early leukemia.

But though it affects more children than any other cancer,

adults constitute the majority of leukemia patients overall.

Once leukemia strikes, the damaged cells reproduce in the blood and the bone marrow

until they take up all available space and resources.

When the bone marrow can no longer produce

the required amount of functional cells,

the blood becomes depleted.

The lack of red blood cells

means that muscles don't get enough oxygen,

the reduced number of platelets is not sufficient to repair wounds,

and the dearth of functional white blood cells impairs the immune system,

increasing the risk of infections.

To restore the normal function of the blood,

leukemic cells have to be eliminated.

But because leukemias are not solid tumors,

they can't be removed surgically.

Instead, the cells are killed inside the body using various treatments

that include chemotherapy,

a combination of drugs that destroys quickly multiplying cells.

Unfortunately, this has the side effect of killing healthy cells,

such as those found in hair follicles or intestines.

And in some cases, the dosage required is so high

that it kills all cells in the bone marrow,

including stem cells.

When this happens, the body is no longer able to create new blood cells on its own.

Fortunately, outside help can come in the form of stem cells

from the bone marrow of a donor.

Once transplanted into the patient,

they rapidly repopulate the bone marrow and the blood.

However, bone marrow transplants are a complicated process

requiring antigen compatibility between the donor and recipient

to keep the transplanted cells from attacking the patient's own cells

as foreign bodies.

Unlike with blood transplants,

there are thousands of HLA types,

and even siblings and close relatives may not have compatible bone marrow.

If this is the case, the search is expanded to a database

containing the genetic makeup of millions of voluntary bone marrow donors.

The more potential donors there are,

the more patients lives can be saved through successful transplants.

Leukemia may be a frightening disease, but there is strength and hope in numbers.