Depending on where you live in the world, a lot of infectious diseases can spread by either wounds on the foot or vectors invading through the sole of the foot.
Considering that the commenter jut told you where in the world he was talking about, I don't think you need to be as generally worried and vague as you're being here.
I mean, depending on where you live in the world, a lot of infectious diseases can spread by either wounds on the hand or vectors invading through the palm of the hand.
GAHHHHH! All the children must wear hand-shoes all the time!! Otherwise they'll all die! Maybe we adults (alleged adults. I refuse to believe I am one.) should too.
The commenter actually didn't say where they lived, and didn't limit the question to any location. I live in Kenya and my one year old walks around a lot of the time without shoes, which is a calculated risk. I assure you I'm being neither silly nor worrisome - I just factually answered the parent's question about what the potential issues are with not wearing shoes.
edit: to be less vague:
(1) Tetanus spores are present in soil all over the world and can easily infect a cut on the foot - between 1 and 2 in 10 cases are fatal (https://www.cdc.gov/tetanus/index.html).
(2) Hookwork larvae are present in soil all over the USA (and many other places) and can infect through intact skin -
not fatal but can have unpleasant complications (https://www.cdc.gov/parasites/hookworm/)
(3) bilharzia infects through skin contact with contaminated water which includes puddles and just wet soil - not in the USA but many other countries -
and kills tens of millions every year (https://www.cdc.gov/parasites/schistosomiasis/index.html)
