According to Wikpedia:

Symptoms: Damage to the bone marrow, by way of displacing the normal marrow cells with increasing numbers of malignant cells, results in a lack of blood platelets, which are important in the blood clotting process. This means people with leukemia may become bruised, bleed excessively, or develop pinprick bleeds (petechiae).

White blood cells, which are involved in fighting pathogens, may be suppressed or dysfunctional, putting the patient at the risk of developing infections.

Finally, the red blood cell deficiency leads to anaemia, which may cause dyspnea. All symptoms may also be attributable to other diseases; for diagnosis, blood tests and a bone marrow biopsy are required.

Some other related symptoms

• Fever, chills, and other flu-like symptoms
• Weakness and fatigue
• Loss of appetite and/or weight
• Swollen or bleeding gums
• Neurological symptoms (headache)

Four major types

Acute vs. chronic

Leukemia is classified into two (acute and chronic) forms.

• Acute leukemia is characterized by the rapid growth of immature blood cells. This crowding makes the bone marrow unable to produce healthy blood cells. Acute forms of leukemia can occur in children and young adults. (In fact, it is a more common cause of death for children in the US than any other type of malignant disease). Immediate treatment is required in acute leukemias due to the rapid progression and accumulation of the malignant cells, which then spill over into the bloodstream and spread to other organs of the body. If left untreated, the patient will die within months or even weeks.

• Chronic leukemia is distinguished by the excessive buildup of relatively mature, but still abnormal, blood cells. Typically taking months to years to progress, the cells are produced at a much higher rate than normal cells, resulting in many abnormal white blood cells in the blood. Chronic leukemia mostly occurs in older people, but can theoretically occur in any age group. Whereas acute leukemia must be treated immediately, chronic forms are sometimes monitored for some time before treatment to ensure maximum effectiveness of therapy.

Lymphoid vs. myeloid

Furthermore, the diseases are classified according to the type of abnormal cell found most in the blood.

• When leukemia affects lymphoid cells, it is called lymphocytic leukemia.

• When myeloid cells are affected, the disease is called myeloid or myelogenous leukemia.

Prevalence of the four major types

Combining these two classifications provides a total of four main categories:

• --->Acute lymphocytic leukemia (ALL) is the most common type of leukemia in young children. This disease also affects adults, especially those age 65 and older.
• Acute myelogenous leukemia (AML) occurs more commonly in adults than in children. This type of leukemia was previously called acute nonlymphocytic leukemia.
• Chronic lymphocytic leukemia (CLL) most often affects adults over the age of 55. It sometimes occurs in younger adults, but it almost never affects children.
• Chronic myelogenous leukemia (CML) occurs mainly in adults. A very small number of children also develop this disease.

The most common forms in adults are AML and CLL, whereas in children ALL is more prevalent.

The exact cause of leukemia is unknown but is influenced by both genetic and environmental factors. Leukemias, like other cancers, result from somatic mutations in the DNA which activate oncogenes or inactivate tumour suppressor genes, and disrupt the regulation of cell death, differentiation or division. These mutations may occur spontaneously or as a result of exposure to radiation or carcinogenic substances and are likely to be influenced by genetic factors. Cohort and case control studies have linked exposure to petrochemicals, such as benzene, and hair dyes to the development of leukemia.

Viruses have also been linked to some forms of leukemia. Certain cases of ALL are associated with viral infections by either the human immunodeficiency virus (HIV, responsible for AIDS) or human T-lymphotropic virus (HTLV-1 and -2, causing adult T-cell leukemia/lymphoma).