When an infection is present, it may spread into the lymph and then drain into the lymph nodes. There, the infection leads to an increase in infection-fighting and inflammatory cells that cause painful swelling of the nodes. This is called lymphadenitis, which can be caused by bacteria, viruses, fungal organisms, and even some parasites. Lymph nodes may also become enlarged as a result of malignant disease such as a lymphoma or metastatic cancer, but as a rule the nodes in such cases aren't painful. Lymph nodes are usually measured in millimeters and are quite small.
Normal nodes feel small and round or oval, like small bee-bee pellets.
It is possible for persistent swollen lymph nodes to be a sign of more serious health problems, such as cancer of the lymphatic system, which is known as lymphoma. Experts say that swollen lymph nodes can indicate the presence of cancer, but that in these cases the swollen lymph nodes are generally hard, fixed, and not tender. Swollen lymph nodes should be checked by a health care professional when a person has swollen lymph nodes and does not recall being sick, when the swelling lasts more than four to six weeks, and when symptoms such as nights sweats, fever, or unexplained weight loss accompany the swelling. This site provides parents with information about swollen lymph nodes in children, including what symptoms should prompt a trip to the health care provider. The Leukemia Society of America is a national health agency dedicated to curing leukemia, lymphoma, Hodgkin's disease, and myeloma.