Try to find the right keywords for your incident. If you are doing the Tuskegee Syphilis Experiment, don't type "Tuskegee Syphilis Experiment". Better to just type: Tuskegee Syphilis
Rather than "American experiments in Guatemala after WWII", just type: guatemala venereal experiments
Sometimes a phrase will help, for instance, "human radiation experiments".
For general articles on the topic, you might try "human experimentation in medicine", since that is a subject term in some of the databases.