The most difficult part of conducting a good search is not figuring out which buttons to push, but rather, in figuring out which terms to use and how to combine them to find literature that is "on topic."
Generating Keywords: Start with your health and/or disease condition. Note possible synonyms and variations.
Disease or Condition:
e-coli 0517 (particular strain)
Controlled Vocabulary: Many databases have some form of controlled or standardized vocabulary. One term is selected as the "preferred" word for describing information and searching. These standardized words and concepts can be called "subject" terms or "descriptors." Words selected for controlled vocabularies are decided by specialists in information science and/or academic disciplines.
MeSH (Medical Subject Headings) are a controlled vocabulary thesaurus created by the National Library of Medicine and used to index articles for MEDLINE and PubMed. Over 24,000 descriptors are arranged in a hierarchical manner called the MeSH Tree Structures.