<unk>  is an estimate of the rate of progress of a disease, based on proportional measures of the extent of<unk>  at different times. Firstly, a proportional measure of the extent of<unk>  is chosen as the disease extent metric. For example, the metric might be the proportion of leaf area affected by mildew, or the proportion of plants in a population showing dieback lesions. Measures of disease extent are then taken over time, and a mathematical model is fit. The model is based upon two assumptions: * the progress of the<unk>  is constrained by the amount of tissue that remains to be infected; and * if it were not so constrained, the extent of<unk>  would exhibit exponential growth. There is a single model parameter r, which is the<unk> . It can be calculated analytically using the formula : where :r is the<unk>  :t1 is the time of the first measurement :t2 is the time of the second measurement :x1 is the proportion of<unk>  measured at time t1 :x2 is the proportion of<unk>  measured at time t2
Apparent infection rate
Infection rate
Risk of mortality
Case fatality rate
Mortality rate
Basic reproduction number
Measured environmental concentration
Epidemic curve