A “Syndrome” is a collection of symptoms of conditions which we suspect means something, but can’t really prove it. It’s contrasted to a “Disease,” which has a definite way of being proved. So for example, people with “Irritable Bowel Syndrome” (IBS) may have abdominal pain, constipation, and/or diarrhea, which can be bothersome or even disabling, but there’s no way to prove exactly what is happening in the body, because all tests are normal. “Inflammatory Bowel Disease” (IBD), however, causes ulcers in the bowel, can be definitively identified by biopsy, and if severe, can require removal of part or even the entire bowel. Occasionally people can die from IBD, never from IBS.
“Acquired Immunodeficiency Syndrome” (AIDS) is also a “syndrome,” and people certainly can die from it. The reason it’s called “syndrome” instead of “disease” was that when first recognized in 1981, all we knew was that some people were suddenly developing a wide variety of fatal infections that had previously been extremely rare. They were caused by “opportunistic” germs that are so weak that they never caused illness. All of use have them in us, but almost nobody ever gets sick because our immune system controls them – unless, of course, the immune system weakens for some reason.
So the term “AIDS” was applied to anyone with “opportunistic infections,” meaning for some reason, they had lost their immune systems. But we didn’t know why, so we couldn’t say it was a “disease.” Then, when a previously-unknown virus was found to be responsible, it was named “Human Immunodeficiency Virus” (HIV), and we do speak of “HIV Disease” (though just say “HIV” for short). Really, AIDS is nothing more than end-stage untreated HIV infection, but we still use the term. By the way, the “A” which stands for “Acquired” is because there’ve always been rare immunodeficiency diseases babies are born with; they’re “congenital” instead of “acquired.”
So back to the term “syndrome,” – syndromes are certainly illnesses, but ones which can’t be precisely defined by biopsies, x-rays, blood tests, or anything. IBS is thought due to disturbances in the rhythm of the bowel, but nobody can prove it. Maybe someday they will.