Late-stage AIDS of the kind you’re talking about makes minor illnesses very dangerous and potentially fatal. (Most modern people with HIV, at least in the developed world, do not progress to the stage you’re talking about thanks to advances in anti-HIV drugs.)
But you would still get a fever. The fact that you don’t have the white blood cells needed to fight the illness doesn’t mean your body can’t detect it. It can still send the chemical signals of tissue damage, and fever isn’t the direct result of white blood cells – it’s actually controlled by a part of the brain called the hypothalamus.
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