Dr. Hibberd's answer:
 

Lymphoma is a blood disorder involving the lymphocyte immune cells (either T cells or B cells or a combination). A massive proliferation of uncontrolled unregulated blood cell production occurs in people who have this disease.

It is often recognized initially as a mass or enlarging lymph node, but sometimes is not recognized until weakness or weight loss is noted. What happens is the body is flooded with immature and often only partially functional cell lines that infiltrate and displace our other organs and tissues, interfering with the function of bone marrow, the lungs, liver, brain and other organs. About 5-10 percent of all cancers are lymphomas, which account for almost 6 out of 10 blood cancers. 
 

Some lymphomas are caused by viruses, but not all. Some are more aggressive than others. Very effective combination treatments (usually chemotherapy and selective radiation treatments) have been shown to be effective for adults and children. These approaches have very high cure rates, but even small treatment delays can be devastating with these usually rapidly growing malignancies.