Epidemiology of Infectious Diseases: Portal of Exit of Agent from the Host (Page 1 of 2) Portal of exit of agent from host -- the path by which an agent leaves the source host. It usually corresponds to the site at which the agent is localized. Therefore, influenza viruses exit the respiratory tract, sarcoptes scabiei in skin lesions, cholera vibrios in feces, and schistosomes through urine. Respiratory tract -- diseases such as influenza, common cold, tuberculosis. Diseases caused by agents using this portal are very difficult to control.