I once owned a car that had an intermittent short-circuit. Some of the time it would work just fine. Other times the whole electrical system would just shut down. It would often happen when I tried to start it, but if I went through a weird, neurotic series of actions - turn on the radio, start the windshield wipers for one swipe, turn off the electric overdrive, turn off the radio, then start the engine - it seemed to start most of the time. It took the service team about 5 attempts to find the short-circuit, but once they found it, the car ran just fine. MS is like that. Instead of one short-circuit, imagine a mouse running loose in the system gnawing at the plastic insulation of the wiring while the service team are replacing the wiring as fast as they can, while simultaneously trying to find exactly what it is that is chewing on the insulation. The mouse is very tricky, it hides so that the mechanics cannot be sure that it really exists - yet.
Our bodies' equivalent of the wiring in your car is our nervous system, which is made up of cells called neurons. The equivalent of the plastic insulation on your car's wiring is a substance called myelin. MS is characterized by areas of the brain and spinal cord in which the myelin has been gnawed on by some unknown agent. The areas of demyelinization and subsequent scar tissue are called scleroses, and there are typically many of them, prompting the name multiple sclerosis.
There are two separate issues here. The first issue is just what the hell is causing these scleroses in the first place? It is still not fully understood, but the current theory is that MS is an autoimmune disease. My immune system is, for bizarre reasons of its own which are unknown to me, crossing the blood-brain boundary and attacking me instead of doing its normal duty. The standard treatments involve either suppressing the immune system, or injecting chemicals that mimic the structure of myelin so that the immune system attacks that instead of me. Current research does not (at the time of writing) provide compelling evidence either for or against the idea that maybe the immune system is going after some currently undetectable invader and the lesions are merely collateral damage in an unseen war.
The second issue is that the scleroses cause a wide range of symptoms depending on where they are located in my brain and spine. Symptoms appear to be caused by blockage and/or short-circuits between adjacent patches of neural tissue. To grossly simplify, neurons, like the wires in your car or in your computer, can be either "on" (sending current) or "off" (not sending it, obviously). MS causes physical trauma to localized patches of neural tissue that can cause the neurons surrounding it to be either (a) always off, (b) always on, or (c) randomly firing, instead of their normal state in which we assume (although our knowledge of the exact mechanism is hazy) that they fire in response to external stimulus or to internal emotional and/or metaphysical states of mind. The scleroses can potentially affect every system of my body that is controlled by my nervous system, which the last time I looked was most all of them. MS symptoms differ from one patient to another, partly because the scleroses will be in different parts of their nervous systems, and the symptoms can even change with time depending on which neurons are shorting out with which. In some MS sufferers the scleroses are permanent, while in others myelin can be regrown. In some of them, there is a constant battle between myelin repair and further demyelinization.
MS is found more often in young people than old, more often in women than in men, and more often in cold climates than warm. Sometimes the symptoms get worse (the technical term is progress), and sometimes the symptoms come and go (the technical terms are relapse and remit). Some people die of MS, some people get progressively worse until they wish they could die, and some have it but never feel a single symptom. To make it worse, some people go back and forth between these states, with the possible exception of the "some people die" part unless you believe in reincarnation (see Religion and Philosophy). Being diagnosed after age 40 is supposedly statistically bad. So is being male with MS. Being both, I am probably (but not necessarily) toast. Being diagnosed before the mid 1990s definitely seems to be statistically bad, so at least I'm safe there.
I occasionally get email from people who dispute my statement in the last paragraph that "some people die of MS". I view arguments to the contrary as specious, but this is only my personal opinion. We all ultimately die of heart failure in the sense that one is not technically dead until one's heart has stopped beating. The cause of death may have been a 9 millimeter bullet to the brain or any number of equally unpleasant alternatives. My mother may have technically died from a chest infection before the terminal bone cancer carried her away, but the cancer was to blame nonetheless. If not for the cancer, her immune system would easily have vanquished the chest infection. If not for MS, some of my friends and acquaintances would not have died from the relatively innocuous things, including pneumonia, starvation, and dehydration, that prematurely killed them. Or we could just stick our heads in the ground and pretend that they died of heart failure.