A new method was suggested to determine the index of deadliness in fixing the intensity of the epizootic process as manifested in its two forms--apparent and inapparent. The method is based on the calculus of probability--a most general formula for the sum of any kind of events. The results obtained have been compared with those reached with the already existing two other methods. The model employed consist of theoretically composed examples corresponding to similar ones in the practice as well as of investigations on Salmonella abortions in sheep. The new method can be employed with all kinds of initial data and results in various variants.