Hey!
I think Health and Human development is an awesome subject! I am in Year 10, and doing Unit 1 currently. So far we have covered simply the Health aspects (Physical, Mental and Social) and the aspects of Development (Physical, intellectual, emotional and social development). Other than this we have looked over impact of nutrients on youth as well as determinants that affect the development and health of an individual. So as you can tell, its really based on general knowledge but just reasoning behind that knowledge. 
It is definitely a subject that can be done without previous knowledge (its only 5 chapters per unit, do it over the holidays if you want). Consider a Year 10 kid doing the subject, and imagine how easy it is.. xD . Also, people say it scales down, but obtaining a study score of 40-50 is not hard by the looks of past results. BUT, you may have some difficulty revising things as there are several definitions etc. Enjoyable overall though.
~got an exam for it on Friday.  
 
  
  
 