When we try to imagine lifestyles and environment changing over time, we often fail to imagine simple things. Consider, for example, Native Americans who lived a pre-mabul lifestyle and had seemingly no viruses and no immunity to them (leading to a disaster when they encountered those).
Some explain that it is partly due to lack of domesticated animals and, thus, lack of viruses jumping between animals and people. If pre-mabul generations were vegetarians, then the same would apply. I may be taking these midrashim too literally, but it is interesting to see how life conditions can change easily.