However, there is also this:
Summary: The cause of MS is unknown but some infective agents are thought to play a contributory role. This review article discusses two of the possible candidates, Epstein Barr virus and Chlamydia pneumonia, and the evidence for each of them.
authors: Giovannoni G, Cutter GR, Lunemann J, Martin R, Munz C, Sriram S, Steiner I, Hammerschlag MR, Gaydos CA.
source: Lancet Neurol. 2006 Oct;5(10):887-94.
I have read the whole article and the piece by Giovannoni especially is very good.