Perhaps you should try some spelling variations:
greatest challenges » greatest challenge, treatment challenges, related challenges
challenges finding » challenges facing, challenges during, challenges arising
finding among » funding among, bonding among, singing among
its greatest » _ greatest
among di » among hiv, among idp, among _
nan yan » nan ya, wang yan, qian yan
yang xi » yang di, yang yi, yang jui
era its » era aims
ng era » 5g era, qing era, deng era

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: ((nan yan) OR (yang xi))ng era its greatest challenges finding among di ;~.
• Adding a wildcard symbol may retrieve word variants: ((nan yan) or (yang xi))ng era its greatest challenges finding among di ;*.