Cockroaches don't go out looking for humans to bite. They go after food and will get attracted to the slightest smell. So if you have not washed your hand with water and soap after a meal, if there are food on your body (because of eating on bed, sofa etc), if there are left overs in your nails, they go after that. Inevitably you get bitten then. The skin gets reddish, stiff and itchy - lasts for upto about a week. You can see two dots, which is the 'bite spot'