Mostly unknown. A few lymphomas are caused by epstein barr virus infection, but in most cases, the cause is not known. Other cancers do not cause lymphomas.
Often unknown. Lymphoma, cancer of lymph system most of the time cause is unknown, some cases definite link to virus infection as in epstein bar virus (ebv) in burket's &hodgkins lymphoma, t cell leukemia virus, HIV infection, genetic dna mutations causing tumor suppression mechanisms. Environmental factors, chemicals, agent orange, herbicides, pesticides are all attributed as cause.