Updated March 31, 2018.
MS stands for Multiple Sclerosis, which basically means "many scars." The scars develop in the central nervous system (CNS), composed of the brain and spinal cord, as the result of an attack from the body's own immune system.
In a normal body, an attack of the CNS would still be a pretty awful thing, but the body would fight the attacking agents, clear away the resulting plaque, then begin to repair the damage. Sclerosis is a type of scar tissue that is atypical of a cleanly healed injury.
In people with MS, the immune system is stuck in fighting mode, commonly thought to be the result of high levels of inflammation in the body. This keeps the CNS stuck in defense mode, preventing normal healing of the damage. The attack is not the result of a typical virus or bacterial infection, it's actually the body's own immune system doing the damage. This process is referred to as "autoimmune" (immune system attacking the self). Dr. Stephen Gundry refers to this process as "friendly fire." There are a hundred or more different autoimmune diseases. Some of the most common ones are Graves' Disease, Hashimoto's Thyroiditis, Lupus, Type 1 Diabetes, Chron's Disease, Rheumatoid Arthritis and Multiple Sclerosis (MS).
Typical symptoms of MS include walking disability, limb numbness or weakness, dysfunctional bodily temperature regulation and/or inability to balance, chronic fatigue, chronic nerve pain, "brain fog", slurring speech, vision problems/pain, erectile dysfunction, and loss of bladder control. An attack can happen anywhere in the CNS, the result debilitating whatever processes the nerves control, often downstream from the actual damage. No two people with MS have the same exact severities of attacks in the same locations, and thus none have the exact same MS or the same exact set of symptoms.
Attacks can do temporary or permanent damage. Women are twice as likely to develop MS, but men are less likely to fully recover from attacks. There are four primary types of MS, Remitting Relapsing being the most common type in people in their early stages.
When the immune system is working properly, it keeps us from getting sick by identifying and fighting pathogens that have entered the body. But how does it identify the pathogens? How does it distinguish pathogens from the thousands of good bacteria types? The immune system is made up of two components: the Adaptive and the Regulatory systems.
The adaptive immune system is responsible for learning what's good and bad, and when identifying something bad, passing that information on to new adaptive immune cells. This is why we only get most illnesses once. And this is how vaccines work; they train the adaptive immune system. When a bad agent enters the body, T-cells in the immune system identify it and responds with inflammation. High levels of inflammation trigger the regulatory immune system's B-cells into attack mode. The regulatory immune system searches for inflammation in the body and starts destroying the triggering agent.
When the immune system becomes overly active, it can become confused and start to attack good agents. This is especially true when the adaptive immune system has learned about a pathogen that is very similar, molecularly, to good parts of the body. The whole thing is a case of mistaken identity for an immune system that becomes aggressive in an abundance of caution. But once the adaptive immune system is incorrectly educated, it doesn't forget it's misinformation. If it has learned to attack good cells in the body it has become autoimmune.
I am not a doctor. I am a scientist (engineer) who has had MS since March 2013.