The diseases that are of the highest risk of accidental intro- duction into farms are denoted by ». These pathogens possess inherent properties of high transmissibility, and have enhanced resistance to inactivation (loss of infectivity) due to environmen- tal temperature and sunlight. Such pathogens therefore tend to occur more frequently on poultry sites. Table 2 summarizes the major route(s) of transmission for each of the major pathogens. Knowing the means of spread of any pathogen is fundamental to the development of a plan of action to prevent spread of the pathogen and outbreak of the disease within a production site. The poultry diseases.