They may require medicines essentially injected into the skin right away (9)
I believe the answer is:
epidemics
'they' is the definition.
Both the definition and answer are plural nouns.
Perhaps they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for epidemics that I've seen before include "Widespread outbreaks of an infectious diseases" , "outbreaks of disease" .)