Hello there...I can understand your concern. If the persons having sexual contact are negative, then it is not possible to either of them to get infection from each other. The virus can't invent itself. It has to be transmitted from someone who has the infection. Sometimes when an individual is in window period, the hiv test can be negative. So repeat the test after 6 weeks just to be safe. And yes there are many bacterial infections like chancroid, gonorrhoea which can be transmitted sexually. Even these can only be transmitted only if the other person is harbouring the infection.