How would one indicate the passage of time in one panel of an illustration?

I am preparing a manuscript, and need to show that the likelihood of contracting an infection is impacted by the duration of exposure of a non-infected person to an infectious one. The image already has an infected person coughing towards a non-infected person. I want to show that the longer the naive person is exposed to the infected one, the greater chance of disease. I’m trying to find a way to indicate this in one frame of an illustration but am finding it difficult to visualize a way to do this. Any help would be GREATLY appreciated.