And don't forget the
microbiome. A big problem with time travel is that you'll run into bacteria you're not accustomed to, and you'll deliver unfamiliar bugs to the destination.
This also applies to meeting uncontacted peoples, like Columbus did in 1492. The devastation ran both ways, where apparently smallpox was traded for
syphilis, and neither side had natural resistance to the foreign bugs. The 15th century syphilis outbreak killed five million Europeans, and ruined the bodies of a much greater number.