Is it plagarism if you are plagiarizing yourself?

In a recent article in Science authors found that approximately 1 in 200 papers published in PubMed (scientific database to the gods) had some indication of plagiarism and that of those papers, approximately 86.2% of the content was similar to previously published work. While plagiarism is a terribly bad thing and should be avoided at all costs, as a person in the academic world I have found myself wondering, are you plagiarising if you plagiarize your own work?

Example: I have now written multiple papers on different aspects of delayed and late testing for HIV among men who have sex with men (granted there is only one in for actual publication at this point), and given that almost all scientific papers start the same way, with an explanation of the broad background of the issue, why the issue is important, and how you plan to look at said issue, I find myself re-writing the same intro over and over and over again. Sure I will update things if new numbers come up and more information, but generally I am just putting the same material down on the page for each paper. If while writing this introduction or in reviewing the existing literature I use the same sentence that I wrote in the first paper I wrote on the topic, is this considered plagiarism?

What do you think fair blog readers, can you plagiarize yourself? and if you can, how are scientists who all study very specific topics supposed to avoid plagiarizing themselves?

Science Article