Since initial exposure to numerous viruses, bacteria and other microbes occurs during childhood, and since viruses are well-recognized as causes of demyelination and inflammation, it is possible that a virus or other infectious agent is the triggering factor in MS. More than a dozen viruses and bacteria — including measles, canine distemper, human herpes virus-6, Epstein-Barr, and Chlamydia pneumonia — have been or are being investigated to determine if they are involved in the development of MS, but none have been definitively proven to trigger MS