You can find a simulator for CTMCs here. It has a documentation on how to write model definitions here.
Use this definition of a SIR model[1] as a starting point.
//SIR Model
const beta = 2e-4[1/h];
const mu = 2e-2[1/h];
const population_size = 1000;
/* Species definitions */
Susceptible();
Infected();
Recovered();
/* The initial state */
Initial: 1 Infected + (population_size-1) Susceptible;
/* Rules */
Infected + Susceptible -> 2 Infected @ beta;
Infected -> Recovered @ mu;
/* Observed variables */
output ##Susceptible;
output ##Infected;
output ##Recovered;Your answer
Model definition contributed by Pia Wilsdorf.