A pathogen is any microbe/micro-organism that can cause disease in a host organism. Pathogens include viruses, bacteria, fungi and plasmodia. Not all microbes are pathogens - only those that cause diseases.
The role of the immune system is to destroy invading pathogens.
There have been many attempts to link pathogens with multiple sclerosis, although, to date, none of these attempts have been successful. It is now generally believed that MS is autoimmune by nature.
However, it remains possible that the initiation of the disease is a result of the specific immune system developing an immune reponse to a section of protein (antigen) from a common pathogen that closely resembles an antigen derived from myelin. Myelin is the insulating material wrapped around the nerves (neurons) in the central nervous system. The theory goes that once this immune pathway is established, the immune system retains it. From time to time it incorrectly recognises the body's own myelin as an enemy and attacks.
The role of viruses in multiple sclerosis
Pneumonia Organism May Play Role in MS Development
Chlamydia not a cause of multiple sclerosis after all (2nd feature)