A model which describes the characteristics of the penetration of the cells by bacteria is presented. Since the process of invasion is preceded necessarily by the step in which the bacteria adhere to the cells, the proposed model is based on the expressions previously derived for the process of adhesion, which allow us to determine the number of attached bacteria under different conditions. Thus, the model considers that invasion occurs irreversibly from attached bacteria to specific receptors located on the cell surface with a rate coefficient = ki so that the invasive capacity in a given bacterium-host cell system is mainly determined by the value of this coefficient. Once internalized, the bacteria can follow three different time courses, namely: 1) intracellular growth is hindered so that the bacteria remain in stationary phase, 2) there is a lag phase during which the bacteria stay in stationary phase before they are able to grow exponentially with a rate coefficient = kc, and 3) the bacteria exhibit a growth exponential phase as they enter the cells. In turn, the time course followed by extracellular bacteria also has a decisive influence on the process of invasion and, in this regard, unbound bacteria are considered either in stationary or in exponential phase. Expressions for these different situations have been derived, and from them, procedures to determine the levels of bacterial infection and for quantitative invasive data analysis are presented.