I read that HIV becomes AIDS when your immune system is so damaged that you have less than 200 CD4+ cells or you get an opportunistic infection. However, there are so many different types of opportunistic infections of varying severities, so how can this be used as a measure of 'disease' (i.e. HIV)progression?
There is some misunderstanding about minor illnesses, such as thrush, that are common in HIV+s, and the serious AIDS-defining OIs. The latter are characteristic of advanced immune damage, and are typically life-threatening if untreated; that is why they are indicators of AIDS.