It may help to answer your question by starting with an explanation about lymphomas in general.
Lymphomas are cancers of the lymphoid tissue which is part of our body's immune system.
Our immune system protects us from infection. It is a complex system made up of the bone marrow, the thymus gland (which lies behind the breast bone), the spleen and the lymph nodes (or lymph glands).
One of the most important cells in our immune system is a type of white blood cell called a lymphocyte. There are two types of lymphocytes: 'B-cells' and 'T-cells'. All lymphocytes are produced in the bone marrow and start life as young, immature cells called stem cells. Some lymphocytes continue their development in the bone marrow or lymph nodes and these are called B-cells but others move to the thymus gland and they are called T-cells.
Many years ago it was thought that lymphomas could be divided into just two conditions: Hodgkin lymphoma (named after Thomas Hodgkin, the London doctor who first described it over 100 years ago) and non-Hodgkin lymphoma (NHL). With the passage of time it has become clear that NHL is not a single illness but includes a number of cancers, which behave very differently.
The description and classification of the various types of NHL has developed over the years as more has been learnt both about the immune system and the cancers themselves. The most recent classification still recognises the difference between Hodgkin lymphoma and NHL but then goes on to divide NHL into some fifteen different tumour types.
The cause for the great majority of these different types of NHL remains a mystery but in three types of the disease, all of which are very rare in the UK, a link with viruses has been established. These are Burkitt's lymphoma, Burkitt-like lymphoma and post-transplant lymphoma.
In 1956 a British surgeon called Denis Burkitt was working in equatorial Africa described an unusual type of lymphoma which was very common in children in that region. This became known as Burkitt's lymphoma. Later research showed that B-lymphocytes in these children became infected with a virus, the Epstein-Barr virus, or E-B virus. Epstein-Barr virus infections are common and usually cause no problems but in central Africa many of the children had chronic malaria infections which reduced their resistance to the virus. In some cases this allowed the virus to change the infected B-lymphocytes into cancerous cells leading to the development of the lymphoma.
In recent years it has been recognised that in the western world there is one type of NHL where the tumour cells have very similar appearances under the microscope to those of Burkitt lymphoma. This rare condition has been called Burkitt-like lymphoma. Further research has shown that a high proportion of patients with Burkitt-like lymphoma (but not all) are HIV positive and many have AIDS. It seems that in this condition once again an Epstein-Barr virus infection occurs and because the HIV has reduced the patient's immunity the Epstein-Barr virus is able to survive and 'transform' the normal B-lymphocytes to cancerous cells.
The same situation has been seen in some patients who have had organ transplants. Often after organ transplantation drugs are given for some time, often years, to suppress the patient's immunity in order to reduce the risk of rejection of the grafted organ. Some of these patients appear to develop E-B virus infections and once again, as their resistance is reduced, this may lead to the development of a B-cell lymphoma.
All three of these virally-related types of NHL behave in a very aggressive way and need immediate treatment.
In the African children with Burkitt lymphoma chemotherapy gives a high cure rate but the Burkitt-like lymphoma and post-transplant lymphomas tend to be more resistant to treatment. Although a variety of different drug combinations have been used cure is not possible in the majority of people. This means that a number of clinical trials are in progress to try and improve the results of treatment.
In conclusion, the likelihood of your husband's NHL being one of those types with a known viral cause is very, very remote. Almost certainly he has one of the forms of lymphoma for which no definite cause is known.