Leukemia is defined as the classification of cancer in which the bone marrow or blood contains an irregular increase of mature white blood cells, which are also known as “blasts.” It serves as part of the broader group of diseases that affect the lymphoid system, blood, and bone marrow. Approximately 30% of children with cancer are diagnosed with leukemia, making it the most common type of cancer seen in children.  90% of the cases of leukemia can be witnessed in adults. 
The first major classification of leukemia is the fact that it appears in both acute and chronic forms:
- Acute leukemia refers to the characteristic in which there is an abnormally fast increase of malignant blood cells that are immature, which inevitably creates crowding in areas like the bone marrow. The crowding that occurs prevents healthy blood cells from being formed, hence making it detrimental to the body system. Acute leukemia requires immediate treatment due to the fast paced accumulation and progression of these immature blood cells, which eventually leaks into the blood stream and affects several organs of the body. This is the most common form of leukemia seen in children.
- Chronic leukemia is leukemia in which there is a massive build up of white blood cells, and these cells are still deemed malignant. Even though the process of the excessive build up can take approximately months or years, these cells are still produced at an abnormally high rate, which results in an irregularly high number of blood cells as well. Unlike the acute form of leukemia, the chronic form is usually monitored for a period of time before an actual treatment in order to achieve maximum effectiveness. This form of leukemia can be seen mainly in older people.
In addition, these forms of leukemia can be subdivided into lymphocytic and myelogenous forms.
- Lymphocytic leukemia, also known as lymphoblastic leukemia, is marked by a cancerous change that occurs within the type of marrow cell that usually proceeds into the form of lymphocytes. Lymphocytes are defined as immune system cells that fight infections. Lymphocytic leukemia tends to also involve B cells, which is a common subtype of the lymphocyte.
- Myelogenous leukemia, also known as myeloid leukemia, is marked by the cancerous change that occurs within the type of marrow cell that proceeds to form red blood cells, platelets, and other forms of white blood cells.
The two classifications listed above enable four central categories of leukemia to be made. In addition to these four main categories, there are also some other rare types of leukemia that is exhibited.
- Acute lymphoblastic (ALL) leukemia serves as the most frequently appearing type of leukemia in children. The survival rates for children tend to be approximately 85% while the survival rate for adults is approximately 50%. Other examples and subtypes are not limited to but include precursor B acute lymphoblastic leukemia, Burkitt’s leukemia, and acute biphenotypic leukemia.
- Chronic lymphocytic leukemia (CLL) typically affects adults over the age of 55, and almost never affects children. Men appear to be more prone to this type of leukemia and the five-year survival rate is approximately 75%. As of now, this type is incurable but there are several treatments that are effective. A subtype of this is B-cell prolymphocytic leukemia.
- Acute myelogenous leukemia (AML) typically occurs more frequently in adult men. It is typically treated with chemotherapy and the five-year survival rate s approximately 40%. Subtypes of this leukemia include acute myeoblastic leukemia, acute promyolocytic leukemia, and cute megakaryoblastic leukemia.
- Chronic myelogenous leukemia (CML) usually occurs in adults. The treatment to CML is typically imatinib, and the five-year survival rate is 90%. A subtype of CML would be chornic monocytic leukemia.
- Hairy cell leukemia (HCL) is often considered a subtype of chronic lymphocytic leukemia, but this kind of classification is very rough. 80% of the affected population are adult men. This type of leukemia is incurable but easily treatable. The survival rate is between 96% and 100% at 10 years.
- T-cell prolymphocytic leukemia (T-PLL) is a rare and aggressive leukemia that typically affects adults. It appears more in men than women. Even though it is rare, it is the most common type of T cell leukemia that has matured. It is not easily treated and the survival rate usually lasts a few months.
- Large granular lymphocytic leukemia involves T-cells and/or NK cells. It is a rare leukemia that is not aggressive.
- Adult T-cell leukemia occurs due to the human T-lymphotropic virus (HTLV), which is a virus that can be relatable to HIV. HTLV infects the CD4+ T-cells and replicates within them.
Causes and Correlations 
Mutations in the DNA typically act as one of the causes of leukemia and other cancers. Specific mutations can then result in leukemia by deactivating tumor suppressor genes or activating oncogenes. These series of events then lead to the disruption of differentiation, cell death, or division.
Viruses may also have an effect on certain forms of leukemia. Retroviruses have been identified as relevant to some cases of leukemia, and the first human retrovirus that goes under this classification is the human T-lymphotropic virus or HTLV-1.
There are cases in which chromosomal irregularities correlate with the greater risk of leukemia. Down syndrome serves as an example that correlates with an increased risk of the development of acute myeloid leukemia, and Fanconi anemia creates an increased risk of acute myeloid leukemia as well.
Signs and Symptoms 
Due to the high number of immature and malignant white blood cells, a high number of normal bone marrow cells are consequentially displaced, which can cause significant damage to the bone marrow. This damage ultimately creates a lack of blood platelets, which creates difficulty for the usual blood clotting process. This signifies the fact that people with leukemia tend to be easily bruised, develop pinprick bleeds, and can have excessive bleeding.
Individuals with leukemia may also have a hard time fighting off simple infections throughout the body, due to the fact that their white blood cells are either suppressed of dysfunctional. Those affected with leukemia frequently experience infection, which can affects body systems that result in sores within the mouth, infection in the tonsils, and diarrhea.
In some cases, the malignant cells of leukemia affect the central nervous system, which results in headaches and other neurological symptoms.
The diagnosis of leukemia is processed by bone marrow examinations and repeated blood counts after acknowledging the common symptoms of leukemia. There are however, some cases in which blood tests do not show indicate the existence of leukemia due to the fact that it is in early stages. Lymph node biopsies are also performed to diagnose specific forms of leukemia.
After a diagnosis is completed, blood tests are utilized to observe the degree of damage that has occurred onto the liver and kidney. X-rays, MRI, Ultrasound, and CT scans are also used to witness visible damage that is caused by leukemia.
- Varricchio, Claudette G. (2004). A cancer source book for nurses. Boston: Jones and Bartlett Publishers. p. 229.ISBN 0-7637-3276-1.
- "SEER Stat Fact Sheets: Leukemia". National Cancer Institute. 2011. ""Approximately 10.8% were diagnosed under age 20""