Many of these, like Guardia, were nonexistent in ancient times, or so localized they didn't have much of an impact. It is our encroachment on large areas that have made them run rampant...as little as 40 years ago, Guardia was Not a problem!

@AnneWimsey These micro-organisms/viruses/and other powerful beasties evolved from earlier forms which killed our ancestors with worms, and other conditions now mutated into equally nasty ones.

@Robecology ask any long-time hiker, they did not used to have to carry filters, purifiers & the like.