Answer:
Option D, the causes of human diseases
Step-by-step explanation:
Most of the diseases are caused by microbes and the remaining are caused by other agents such as animals bite, etc.
Specific microbes causing specific diseases affects always remains the same. For example if salmonella Typhi causes typhoid diseases, then causative agent will remain same. However, the way our body responds to such microbes may change in long run.
Thus, the symptoms of a disease may change, the number of people effected by a disease may change and even the the understanding of how diseases affect people may change but the causative agent i.e cause of human disease remains the same.