Cancer is a class of diseases in which a collection of cells display uncontrolled growth, invasion and sometimes metastasis. These three hateful properties of cancers discriminate them from benign tumors, which are self-limited, and do not etastasize. Most cancers form a tumor but some, like leukemia, do not. The branch of medicine anxious with the study, diagnosis, treatment, and avoidance of cancer is oncology.
Cancer affects people at all ages with the risk for most types increasing with age. Cancer caused about 13% of all human deaths in 2007
Cancers are caused by abnormality in the genetic material of the transformed cells. These abnormality may be due to the effects of carcinogens, such as tobacco smoke, radiation, chemicals, or infectious agents. Other cancer-promoting genetic abnormalities may haphazardly occur through errors in DNA replication, or are inherited, and thus present in all cells from birth. The heritability of cancers is usually affected by complex exchanges between carcinogens and the host's genome.
Systemic symptoms: weight loss, poor appetite, fatigue and cachexia ,sweating, anemia and specific paraneoplastic phenomena, i.e. specific conditions that are due to an active cancer, such as thrombosis or hormonal changes. Some cancers can be caused by infection. This is especially true in animals such as birds, but also in humans, with viruses responsible for up to 20% of human cancers worldwide.