More than 100 clinically distinct types of cancer are recognized, each having a unique set of symptoms and requiring a specific course of therapy. There are four groups in a major categories.
Abnormal numbers of white cells (Leukocytes) are produced by the bone marrow.
Abnormal numbers of lymphocytes (a type of leukocyte) are produced by the spleen and lymph nodes.
Solid tumors grow from derivatives of embryonal such as connective tissue, cartilage, bone, muscle and fat.
Solid tumors grow from epithelial tissues, the most common form of cancer; epithelial tissues are the internal and external body surface coverage and their derivatives and thus include skin, glands, nerves, breasts, and the linings of the respiratory, gastrointestinal, urinary, and genital systems.
Cancer has major characteristics, such as Metastasis:
It is the ability of a malignant cell to detach itself from a tumor and establish a new tumor at another site within the host.
In the early 1900s, both leukemia and sarcomas in chickens were shown to be caused by viruses.But for many years these discoveries were not considered relevant to the cause of human cancer, Furthermore, human cancer did not appear to be infectious, and there were no confirmed an etiologic virus.
Both RNA and DNA viruses have been found capable if inducing cancers in animals; in these animals the affected cells are transformed, resulting in the formation of tumors.such tumor-inducing viruses are called oncogenic viruses.
Oncogenes, Retroviruses and cancer:
Oncogenes (from oncogenes, the greek word for mass or tumor) were originally defined as the genetic elements (genomic sequences) of retroviruses responsible for the malignant transformation of host cells.