If a person gets an "AIDS-defining illness," this is usually a sign that the person has AIDS. Healthy people do not get these illnesses, because a healthy immune system is strong enough to fight off these diseases. Because of this, getting an AIDS-defining illness is a sign that a person's immune system is seriously damaged. In a person with HIV, getting an AIDS-defining illness signals that the HIV has damaged the immune system badly enough that the person now has AIDS.