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
4.1 greatest » 4.1 treatment, _ greatest, its 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 4.1 » era 2.1, era 3.1, era 1.1
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 4.1 greatest challenges finding among di ;~.
• Adding a wildcard symbol may retrieve word variants: ((nan yan) or (yang xi))ng era 4.1 greatest challenges finding among di ;*.