What about parameters that are hard to measure? Take for instances infectious disease models, often a very simple set of ODEs that are capable of capturing the

What about parameters that are hard to measure? Take for instances infectious disease models, often a very simple set of ODEs that are capable of capturing the wide ranging dynamics that we observe. One of the crucial parameters in these models is the transmission parameter, which determines the rate at which susceptibles become infected upon contact with an infected. Transmission is a function of a long string of physical, chemical, biological, social and economic variables. I am not sure how feasible it is to determine from first principles this parameter and so it is obtained from fitting the model to data. So in this case, one does not directly measure the parameter, but infer it from the data conditional on the model.