It's amazing how much of our digestive system isn't known.
Having had a bad flu years ago, it had a life altering impact on my own system, where I now have to be very particular about what I eat, because I lost so much of that good bacteria.
This in turn has an impact on my SO, since she eats mostly what I eat. My son has a bit more diet diversity, since I worry he'll end up in a similar condition.