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.
To follow up on some other comments. Late Stage AIDS infection is when most of your body’s CD4 T-cells/ white blood cells are compromised and you don’t have much of an immune system. The smallest infections can be fatal at this point, but due to modern anti-HIV retroviral therapy most people don’t actually progress to full blown AIDS.
So as long as a person is on anti-retroviral therapy, they’ll just get sick, they’ll still have a fever, running nose, aches, etc. They might still need to go to a doctor and get anti-biotics or other medicine if their HIV went a while without being detected.
Now when it comes to full blown aids, the body still has some T-cells that are being produced, but the HIV virus is actively attacking them as they’re being made, so a person with late stage AIDS may have signs and symptoms of an infection, but will very rapidly go down hill as the infection spreads and the body can’t produce more T-cells to stop it
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