Ya, happens all the time, I've seen them with holes big enough to fit a few fingers, lol. Fix is a but of work, it doesn't look too bad, I'd say get a good dripping oil spray, that will stop it in its tracks.
Is it rotted through or just surface rust. If it is surface rust clean it up with a wire wheel, put down some red brown primer, and paint. If it's rotted cut it out and replace or it will come back. Rust never sleeps.
My buddy Adam cut it out and replaced it. While he was in there he sleeved the area for the k-member bolts so that there was no way for it to squish in case it rotted some more or was worse than he expected, or his welding just sucked, haha. That car is gone now, so I can't tell ya how it held up.
If it's just surface rust take a drill with a wire wheel and remove all the rust in the area and around the area so that you're down to good metal again, prime and then paint or as suggested above Por 15.
Or cut out the bad rust, clean the area with drill and wire brush the add new metal where needed and do the same to by welding up any small spots if you have a welder.