2 guides on this topic
epidemiological-measures
What each epidemiological-measures question is really asking, how to structure the answer, and where the marks are likely to sit.
Equal case counts in unequal groups, which is what makes the risk ratio worth calculating. Then the harder question: whether the pool is a plausible route for this organism at all.
Two datasets with dates that matter. Prevalent cases stay out of the incidence denominator, and person-time has to be accumulated subgroup by subgroup rather than assumed.