Host-pathogen models are essential for designing strategies for managing disease threats
to humans, wild animals and domestic animals. The behaviour of these models is greatly
affected by the way in which transmission between infected and susceptible hosts is
modelled. Since host-pathogen models were first developed at the beginning of the
20th century, the 'mass action' assumption has almost always been used for transmission.
Recently, however, it has been suggested that mass action has often been modelled
wrongly. Alternative models of transmission are beginning to appear, as are empirical
tests of transmission dynamics.