͑1͒ Susceptible class s͑t͒ consists of those individuals who may contract the disease. ͑2͒ Exposed class e͑t͒ consists of those individuals who have been infected by the disease but are not yet infectious. ͑3͒ Infectious class i͑t͒ consists of those individuals who are capable of transmitting the disease to susceptible individuals. ͑4͒ Recovered class r͑t͒ consists of those individuals who are immune to the disease.