With multiple sclerosis (MS) and other autoimmune diseases the immune system mistakenly attacks the body's own tissues. In the case of MS, it damages myelin, the protective covering of nerves in the central nervous system. This interferes with conduction of nerve impulses, leading to such symptoms as losses of vision, motor strength, coordination and bowel and bladder control. Depending on what type of MS you have, symptoms may come and go or progressively worsen. Drug treatments for the disease suppress the immune system so that flare-ups of autoimmunity become less likely.
Echinacea, from the dried root and leaves of the purple coneflower, Echinacea purpurea and related species, stimulates the immune system. It is used to treat common colds and other minor infections. Some people take echinacea to enhance immunity in the absence of infection.
When you have MS, getting a cold is more than an uncomfortable nuisance. Viral infections can trigger flare-ups of the disease so you have to be pretty careful about how you deal with them. I think it is okay for MS patients to use echinacea on a short term basis for the duration of a cold or other minor infection. But I wouldn't recommend using it on a long term basis - we simply don't know whether the way echinacea affects immune function impacts the potential for autoimmunity. (This same caution applies to other immune-enhancing herbs such as astragalus).
You might be interested to know what the National Multiple Sclerosis Society has to say on the subject: "...echinacea may stimulate the immune system. This is a theoretical risk for people with MS because their immune systems are already inappropriately stimulated. No study has been done to investigate this theoretical risk in people with MS."
Andrew Weil, M.D.