But Ancient Rome experienced a much more notable jump in mortality among children and young adults in the hot summer months, a pattern we do not see today. The spike is almost definitely the result of gastrointestinal bugs that enter the body via water or food contaminated with feces. This happens more often in the summer because the bugs reproduce at a faster rate in warm weather. They disproportionately affected infants and new arrivals from the countryside who hadn’t yet developed immunity.