* Agent of disease 
          * Being the carriers/vectors of disturbing , destroying or causative agents 
            * vectors being 
              * any other species which carry the disease with in themselves
                * insects
            *  causative agents 
              * non - cellular life 
                * Viral Infection , virology 
              * cellular life 
                * Bacterial Infection , microbiology 
                * Parasitic Infection , parasitology
                * Fungal Infection , mycology 


   * Entry of either vectors or causative agents 
    * Mode of transmission of either vectors or causative agents
      * water borne disease 
      * air borne diseases
      * food borne diseases
        * Minerals and nutrition 
    * contact with other hosts 
      * physical contact or sexual contact