General models incorporating search behaviors are used to demonstrate the parallels between attack rates in host-parasitoid systems and transmission processes in sexually and vector-transmitted diseases. Density-dependent transmission, in which the probability of an individual's becoming infected is a function of the density of infectives, I, is the usual assumption in disease models. Frequency-dependent transmission, in which the probability of an individual's becoming infected is a function of the proportion of infectives, I/N, is often considered characteristic of venereal and vector-based systems. These two characterizations of the transmission process are shown to represent extremes of the Type II functional response curve. When there is vector-based transmission, and depending on the details of vector behavior, the probability of an uninfected host's becoming infected may range from being predominantly a function of I to being proportional to I/N2. With a limited number of hosts visited per vector, transmission may decline with increasing overall density of the host population; this was observed in empirical data for a pollinator-transmitted disease. Unified, general models of the transmission process are essential for comparison of dynamic processes in different systems and for studies of the evolution of the transmission process itself.