Professor Paul W. Ewald proposed stealth infections as the cause of cancers and heart disease in his popular 2000 book, Plague Time. At that time, he reported that less than 5 percent of human cancers are known to be caused by something other than an infectious agent. The role of viruses in causing some cancers is well established. For cervical cancer, we already have a vaccine available. For many other types of cancer, infectious agents are suspect. Most of these infectious agents are viruses, but bacteria, fungi, and protozoa are also possibilities.
Non-Hodgkin's lymphoma is a good example of a cancer type under investigation for its relationship to infectious agents. This is a cancer that begins in the white blood cells of the immune system. It usually starts in a lymph node, and may spread to almost any part of the body. There are many different subtypes. The first step in determining whether a cancer is caused by infectious agents is detection of the agents in patients. For non-Hodgkin's lymphoma, many candidates have been identified. The primary ones are:
1. Epstein Barr virus, associated with multiple subtypes, including lymphocyte disorders in patients with AIDS or organ transplants
2. Human Herpesvirus 8, associated with multiple sustypes
3. Human T lymphotropic virus type I, associated with T-cell leukemia/lymphoma
4. HIV, associated with non-Hodgkin lymphoma in patients with AIDS
5. Plasmodium falciparum, associated with Burkitt's lymphoma
6. Hepatitis C virus, associated with multiple subtypes
7. Helicobacter pylori, associated with gastric non-Hodgkin lymphoma
8. Campylobacter jejuni, associated with small intestine non-Hodgkin lymphoma
9. Chlamydia psittaci, assocated with non-Hodgkin lymphoma of the eye
10. Borrelia afzelii, associated with non-Hodgkin lymphoma of the skin
The next step is experiments on animals and isolated human cells to identify the mechanism(s) for the infectious agent to produce the cancer. Three mechanisms have been described for non-Hodgkin lymphoma. Epstein Barr virus, Human Herpesvirus 8, and Human T lymphotropic virus directly transform lymphocytes into potentially malignant cells.