Fun fact: tetanus comes from dirt, not rust. But it has to get into your blood and typically that only happens when you get pierced by a piece of dirty metal, which will typically be rusty if it's in that situation.
Yeah but Tetanus is caused by a specific bacteria called Clostridium Tetani and I don't know how it would find its way to a random thumb tack, inside a building.
Correct. If you take a totally clean, disinfected nail, stick it in some lab grade brine until it gets rusty, and then stab yourself with it, there is a 0% chance of getting tetanus.
You could technically have tetanus on something without it being covered in dirt. My point is tetanus bacteria lives in the dirt. But the rust has nothing to do with it other than making the metal more likely to cut or pierce you.
It appears to be a complete US phenomenon to me. I have only ever heard this myth in online media, and it just seems absurd iron oxide somehow spawns Bacteria.