Perhaps you should try some spelling variations:
greatest challenge » greatest challenges, great challenge, treatment challenges
challenge funding » challenge finding, challenges finding, challenge facing
funding among » bonding among, building among, jumping 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
rda 4.1 » ra 4.1, area 4.1

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 rda 4.1 greatest challenge funding among di ;~.
• Adding a wildcard symbol may retrieve word variants: ((nan yan) or (yang xi))ng rda 4.1 greatest challenge funding among di ;*.