HIV attacks a specific type of immune system cell in the body. It's known as the CD4 helper cell or T cell. When HIV destroys this cell, it becomes harder for the body to fight off other infections. When HIV is left untreated, even a minor infection such as a cold can be much more severe.
Many people who get the flu recover in under two weeks because the immune system is able to clear the virus, leaving no trace of it in the body.
basically hiv attacks the immune system and the flu doesn’t really stand a chance against our immune system