STIs and HIV transmission
The presence of an inflammatory STI (chlamydia or gonorrhoea), or an ulcerative one (genital herpes, syphilis, etc) increases the risk of getting, or passing on, HIV, even when there are no symptoms.
In people with HIV, STIs can increase your viral load in your blood, anal mucus, pre-cum and cum, as well as in sores and lesions, which increases the chance of passing on HIV. They can also decrease your CD4 count. HIV is also more concentrated at the sites of STIs. 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 via STI sites more easily due to clustering of white blood cells at these areas.
Mucosal damage caused by STIs promotes both HIV transmission and acquisition.