I have yet to have one die or get infected, but then again I also invest in Iodine.
For me the diffrance in the Cornish crosses is I have to buy them. The Large breed birds like my Barred rocks come with hartching season, They are vertually free. So while I can pay .89 -1.00 a bird from the hatchery, then have to brood the chicks myself, buy feed etc.. for a bird that grows a bit faster.
I prefer the free birds that I don't have to take care of, they have a mother, I don't have to buy feed for, they run on open range, and all I have to do is make one tiny slit and pull the trigger on my iodine bottle.. and poof.. Not only that but the Capons then will brood more chicks, actually doubling my chicks produced as the Capons will act as hens while the real hens keep on laying.
Yes.. I could buy a incubator, and comertial meat birds, but I would rather spend my money on things I can't get for free.