You could get sick from being barefoot especially outside if there are parasites and bacteria that can enter your skin.
Some parasites can enter your skin which are worms and then there could be worms in your feet.
However it's rare that you would get worms or get sick from being barefoot.
I go barefoot a lot in my own yard and so do my kids when they are playing and it's hot outside.
There's nothing on the ground that would hurt their feet and so far they have never gotten sick from being barefoot.
I used to go barefoot as a kid nearly everyday in our yard when it was arm and I never wore or wear shoes in the house.
I've never gotten any parasites or worms or other sickness from being barefoot.
My kids even play in the mud barefoot.
My 2 year old I strip him down to just his diaper and he goes around in our fenced in back yard and plays in a mud pit his dad made for him.
He's always barefoot doing that.
You can get sick from a lot of things but you're very unlikely to get sick from being barefoot.