The two state variables sick and healthy depend on each other. An increase in the number of sick people automatically results in a decrease in the number of healthy people. It is proportional to the number of sick people (the more sick people, the easier it is to be infected) and proportional to the number of healthy people (the more healthy people, the more can be infected), i.e. proportional to the product of sick and healthy people.

SickIncrease is proportional to Sick*Healthy.

If you take the unit hours, the starting value of the rate of change and from this the proportionality factor is easy to determine.