STIs and HIV transmission
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 are already HIV positive (you have HIV), STIs can increase your HIV viral load. HIV viral load means the amount of HIV in your blood, anal mucus (the lining of your arse), pre-cum, cum (semen), and vaginal fluids, as well as in sores and lesions. If you have HIV and also have an STI, this can increase the chance of passing on HIV to other people too. STIs can also decrease your CD4 count.
CD4 cells are an important part of your immune system. They protect you by finding germs and sick body cells and killing them.
If you have an STI as well as HIV, then both the STI and HIV can be easier to pass on to your partners.
In people who are HIV negative, HIV can enter the body more easily in the area where you have the STI (e.g. your dick, your arse, your throat or your vagina).