Perhaps you should try some spelling variations:
"complicated diseases" » "mediated diseases"
"complicated case" » "complicated acute", "complicated chapter", "complicated grief"
"complicated cases" » "complicated acutes", "complicated chapters", "complicated griefs"
"communicable diseases" » "communicate diseases"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (("communicable diseases") OR ((((((("complicated diseases") OR ("complicated case"))) OR ("complicated cases"))) OR ("complex diseases")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"communicable diseases\") or (((((((\"complicated diseases\") OR (\"complicated case\"))) or (\"complicated cases\"))) or (\"complex diseases\"))))".
• Removing quotes may allow a broader search: (( communicable diseases ) or ((((((( complicated diseases ) OR ( complicated case ))) or ( complicated cases ))) or ( complex diseases )))).
• Adding a wildcard symbol may retrieve word variants: (("communicable diseases") or ((((((("complicated diseases") OR ("complicated case"))) or ("complicated cases"))) or ("complex diseases"))))*.