I looked at the linked study. It's not very complicated. In fact one could argue their method was too simple. They estimated the number of illnesses by dividin

I looked at the linked study. It's not very complicated. In fact one could argue their method was too simple. They estimated the number of illnesses by dividing the number of hospitalizations by a series of probabilities (the probabilities constitute a funnel from someone getting an infection to someone being hospitalized - for example the probability you actually seek hospital attention, the probability your test accurately detects influenza, etc). The probabilities were uniformly sampled from a range of plausible values (derived from their survey data). They did this process 10k times to arrive at an distribution over the number of illnesses (this is a Monte Carlo simulation).