Disease frequency is used to measure the situation of the disease with reference to the population size and time period
which is in a fractional form. The lower part of the fraction, known as denominator is the important part as it was used
to calculate a rate or ratio. Since the disease frequency is based on a ratio estimator, the results are highly dependent
upon the value of denominator. Therefore, the main aim of this paper was to propose a new method in calculating the
denominator for the relative risk equation with the application to chikungunya disease data from Malaysia. The new
method of calculating the denominator of the relative risk equation includes the use of discrete time-space stochastic
SIR-SI (susceptible-infective-recovered for human population and susceptible-infective for vector population) disease
transmission model instead of the total disease counts. The results of the analysis showed that the estimation of expected
disease counts based on total posterior means can overcome the problem of expected counts estimation based on the total
number of disease especially when there is no observed disease count in certain regions. The proposed new approach to
calculate the denominator for the relative risk equation is suitable for the case of rare disease in which it offers a better
method of expected disease counts estimation.