what i mean is what symptoms/illness can cause condemning, and how that be prevented?
You are unlikely to get a whole carcass destroyed, however you may not get parts back eg liver with worms, joints where a trauma has happened etc.
In terms of prevention I would suggest the following - which I'm pretty sure most on here would do as standard in any case :
Buy from known source (eg not market where you don't know history)
Ensure wormed by breeder, or worm yourself, or do worm counts
Isolate new stock from existing for a period of time to ensure you don't bring illness into the general herd. This can be from full isolation unit for a month if you are a regular breeder to just keeping new ones away from existing ones for a couple of weeks to check that they are healthy before mixing
Watch your pigs daily rather than just throw food at them, watching is the best way to pick up when pigs are in not well, and knowing your pigs will tell you that they one that is always first for food is now lagging means something not right. Isolate sick pigs quickly before illness spreads, and treat promptly - if you don't know what's wrong treat symptomatically in conjunction with phone calls to vet. If bad get vet out rather than hope.
Now of course you don't have to do any or all of the above, and you can get a pair of cracking weaners from a market, not worry about worms, throw them in you herd, chuck food at them and immediately go back inside to watch "strictly" , and end up with fantastic meat on the table, the above all just change the risk that something will go wrong.