The mystery was, "How is the disease Malaria transmitted from person to person?"

What Ross actually discovered was that
Malaria is transmitted by the Anopheles mosquito whose bite injects saliva containing sporozoites of the malaria-causing protozoon parasite Plasmodium into the human victim's bloodstream.
However, it was not until 1948 that all the stages in the Plasmodium life cycle were identified and fully understood.