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
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 3.1, era 1.1, era 5.1
era 2.1 » era 3.1

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 ((era 4.1) OR (era 2.1)) greatest challenge funding among di ;~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((nan yan) or (yang xi))ng ((era 4.1) OR (era 2.1)) greatest challenge funding among di ;~".
• Adding a wildcard symbol may retrieve word variants: ((nan yan) or (yang xi))ng ((era 4.1) OR (era 2.1)) greatest challenge funding among di ;*.