There are a lot of things people do that contribute to decreased exposure to microbes. One of the biggest things we do is sanitize and clean. 100 years ago people lived dirtier, which is a bad thing but it did help their exposure to microbes. Children played outside and there were more manual labor jobs that exposed people to germs, dust and dirt. This diversified the microbial community.
Another way is to have a c-section. This may seem small but children born via c-section are exposed to way less microbes and usually fall behind in their diversity and health.
People also eat a lot of packaged and processed foods. If your diet doesn’t include probiotics, fermentation foods, yogurts, or is high in red meat and fats then you will not be getting the microbes that you need.
Finally, children that are not breast fed usually have a less diverse and less healthy microbial community. There is not much study that I have observed on this topic but there is a strong correlation between healthy microbes and breast fed children.
