Not just pathogens hitch rides on footwear. When I was a ranger in Alaska part of my job was giving presentations on interpretive hikes. At the end of a climb to a scenic overlook I chose to talk about the threat of invasive species. One of the visitors remarked it was strange I would bring that up when we were looking at such a pristine wilderness environment. "You think? Look over there. That's pineapple weed and thistle, both invaders. How did they get here? Hikers' boots or their dirty tents." Every year when I return from hunting in Montana I make sure the dogs are brushed clean, my boots are clean, and, most importantly, my vehicle gets a thorough washing before I leave, especially the underside and wheel wells. There's some nasty invasive shit growing on the range there that I don't want to bring back (hopefully it's not already here).
By the way, ever notice the gigantic prickly pear in RSA? Yep, not native. Our gift to Africa. It was actually imported on purpose during the colonial era to use as fencing and for emergency livestock forage during droughts. But the shit took over and created a huge problem. Eventually another exotic was introduced: a moth that, as I recall, laid eggs on the plant and its larvae bored inside. Currently it's estimated prickly pear has been reduced 700% from 1950's ... and there's still plenty of it remaining. Must have really taken over. Agave cactus/succulent was also introduced in a failed attempt to build a tequila industry. I was told the Mexican govt sold SA the wrong species plant on purpose and the product tasted bad. The plants remain but do not appear to have spread like the prickly pear.