Question

why is AIDS known as a syndrome

Answer 1

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AIDS :  Acquired Immuno Deficiency Syndrome.
It is called as syndrome because its cause due to collection of symptoms .
 And it is due that our immune system become so weak that it is vulnerable to any pathogen.
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Answer 2

AIDS stands for Acquired Immune Deficiency Syndrome.

It is known as syndrome because it makes our immune system weak and thus reduces our immunity against diseases as a result of which even common diseases turn into severe one and lead to our death. So, death isn't caused by AIDS but due to the severity of the disease which the person was affected with, that's why AIDS is called a syndrome.