Figure 1. Markov models of the natural history of active trachoma.

A) A simple model of infection with a constant hazard of becoming infected λ or recovering from infection ν. This same two-state model can be used to describe the process of the onset and resolution of inflammatory disease. B) A four state model of infection and disease with states indexed from 1 to 4. In this case transition intensities for the infection and disease processes are mutually dependent. For example, the probability of clearing infection depends on whether inflammatory disease is present. The ‘standard’ model of trachoma natural history corresponds to the solid arrows with transition parameters marked. This is illustrated in C), where time runs from left to right and disease states are indicated at the top of the figure.