Nice to have a good data-based take on this question make it to the front of HN!

One of our better microscopes these days is DNA sequencing, especially for cancer, and the particular base mutations and the sequences in which they occur give heavy clues about the types of mutagens that are going on. The DNA damage from UV radiation from the sun and bulky adduct repair from smoking damage are vastly different. Even when cells have a defect in a repair mechanism, you can tell which repair mechanism is broken based on the particular base changes in which context.

A study from 2025 reapplied these Alexandronv signatures to colorectal cancer with a global set of cohorts, and suggests that colibactin, a mutagen produced by some strains of E. coli and related bacteria, could be driving some of the increase in early age colorectal cancer:

https://www.nature.com/articles/s41586-025-09025-8

Of course we don't know exactly how much of the increase, or the other explanations; causality is multi-causal and I bring this particular cause up because it's one of the stronger leads so far. But when we've lost our keys in the night, even if its easiest to look under the light of the streetlamp, that doesn't mean its the only place we might find them.