The hydrazine-nucleated infectious development process was studied kinetically by examining the time proceeding of the conventional development and the infectious development. In describing the process, two characteristic factors are noticeable. One of them is the threshold level of the conventional development to occur the infectious development, and the other is the induction period for starting the infectious development. We discussed some factors influencing on them, for example, the nature and the content of the hydrazine derivatives in the AgX emulsion layer, or, an adsorption of an anionic cyanine compound on the AgX.