It is crucial to ascertain the epidemiological characteristics of emerging infectious diseases with pandemic potential through the investigation of the First Few X (FFX) cases and contacts, coupled with the estimation of epidemiological parameters. The primary objective of FFX investigations is to gather epidemiological, clinical, and virological data from a limited initial pool of laboratory-confirmed cases and contacts, aiming to delineate the actual transmission dynamics of a novel pathogen. These identified characteristics from FFX investigations can yield epidemiological parameters indicative of the transmissibility and severity of infectious diseases. Moreover, they can inform the development of targeted strategies for disease prevention.