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

Maybe a little time bomb with its clock ticking?

How about showing the hands of a clock moving.

A little graph line to the side, time vs. risk? It can be exaggerated, with the words on the axes very large and a bold, coarse graph line without any scale markings. I think anyone who’s seen a graph will know how to interpret it.

I always do a black panel with white lettering saying, “Later that day…” or “One week later…”.

Maybe you could have one large image of the infected person coughing at the other person, and cartoonize the “disease” into something visible that is moving (show air streams, motion blur, or something) towards the non-infected person. Then have some (maybe 3) smaller cartoon-like frames on the side of the non-infected person that show more and more of the “disease” landing on the non-infected person as each cartoon frame progresses. So maybe there will be a few airborne “disease” vectors in the overall main image between the two people, but closer to the infected person, then they move closer in the first cartoon frame, some land in the second frame while others remain airborne, and in the final frame the non-infected person is now covered in the “disease”. If you’re good, you might be able to illustrate (literally) the motion and time passing in the way you draw the “disease” in the main frame (motion blur, lighter and darker colors to emphasize different stages in time, etc) so that you could do away with the cartoon block.

If it’s by the minute, divide a panel up into 4 or 5 small panels, and show a clock in each one, with movement squiggles behind the hour and minute hands and each one showing a later hour.

Or if it’s by the day, then instead of showing clocks in those 4 or 5 panels, instead show a day-by-day calendar. The kind where you tear out the page each day. And show that, with a human hand coming in from the corner holding yesterday’s torn page along with it.

Between the infected person on the left and the uninfected on the right, make a little horizontal scale [bright colored bars, maybe red] indicating time periods. At the end of each scale put the % likelihood of infection On the right side show the non-infected person.

You could do multiple trace-overs of the non-infected person gradually getting sicker (and slouching over, so you’re not doing the same sketch directly over a previous trace-over). Also, as the first two posters suggested, you could have the hands of a clock moving as well.
Good luck, what you’re doing seems like quite the challenge.
brain… melting D: