An infected non-union is long standing, in other words, the bone has had a previous fracture, the bone has not healed, maybe one of the reasons it didn’t heal is because there’s a low grade infection, a low grade colonization into the bone or around the wound that’s preventing that bone from healing.
It’s different from an infected fracture, which occurs acutely. An infected non-union smolders along, may not be manifest, but you may just note that the fracture hasn’t healed and then once you start to treat it you discover that not only has it not healed, but it’s also infected.