Use
Boolean Operators:
and -
combines search terms so that each search result contains all of the terms. For
example, immunization and infant finds articles that contain both "immunization" and "infant."
or -
combines search terms so that each search result contains at least one of the
terms. For example, shots or vaccinations finds results that contain either "shots" or "vaccinations."
not - excludes
terms so that each search result does not contain any of the terms that follow
it. For example, shots not guns finds results that contain "shots" but
not "guns."
Search Strategies 
