If you have an STI it can increase the risk of getting, or passing on, HIV (the virus that can lead to AIDS), even when there are no symptoms.
If you do not have HIV and have unprotected sex, HIV can enter the body more easily through sores or blisters and in the area where you have the STI (e.g. your penis, bum, throat, or vagina or front hole).
If you are already HIV positive (you have HIV) and not taking medicine to treat HIV (treatment), STIs can increase your HIV viral load. HIV viral load means the amount of HIV in your blood, pre-cum, cum (semen), rectal (anal) fluids, and vaginal or front hole fluids, as well as in sores and lesions. This can increase the chance of passing on HIV to other people. It can also be easier to pass on an STI if you have HIV and not taking treatment. If you are HIV positive and not taking treatment, HIV damages your immune system and makes it hard for your body to keep itself well, and having an STI can also stress your immune system more.