Can someone tell me what the chances are of being infected with aids/hiv while spending time in prison?

No. I don't think there have been any studies looking at the risk of HIV infection in prison settings. As long as you avoid contact with blood or semen the chances are zero. For instance, if you are in solitary confinement you have nothing to worry about! other than that, avoid sex, tattoos, and IV drug in jail and the risk is zero.