last year we had four weaners, one of which became ill shortly after arriving with us. It was runty, wheezing, shaking, and kept stumbling off its legs. It was diagnosed with pneumonia and treated accordingly. At the time my farmer friend said the damage to its growth (i.e. stunted) was done and it would never catch up to the others. I still wanted to try so it had a course of antibiotics and after a lot of care it got better.
However, my farmer friend was right, it never caught up, stayed tiny and when it came time to slaughter the others this small one was hardly worth the bother! It went in our freezer so it's not a huge deal but in hindsight I would have pts rather than throw so much money at. I'm not saying this happens in every case but that has been my experience.