What is the main challenge faced by biologists researching infectious diseases?

Well there alot of challenges faced by biologist researchers during the research of infectious disease, however major of those include the followings:
Defining and measuring the fitness for pathogens across the scales, developing models to capture the impact of co-infection on the evolutionary process, modeling how pathogen characteristics shape the evolution of hose immune diversity, understanding the maintenance of pathogen diversity, and developing the better models for the impact of genetic systems on pathogen evolution .

A lot of challenges u