Salmonella. If you vaccinate your chickens against Salmonella you can just keep them at room temperature with no significant risk. Some countries require by law that all their chickens need to vaccinated, the US doesn't.
Vaccines would only cover the last 200 years or less.
My guess is the increased distance between farm and table. When people kept there own chickens they were exposed to these pathogens on a routine basis giving them a natural immunity. The strains of those pathogens were also not as virulent as the ones today. A consequence of antibiotics and the sanatation practices required in factory farms has killed off the weeker varieties.
The most effective selective breeding program is any incomplete sanitation process.
Vegas was overrun with Elvis impersonators after he died, but they were around before he died to. One of the best, and the only one Elvis liked and approved of, was Andy Kaufman. Here his is on Carson, it's an amazing 6 min. He had E nailed down cold.
"It's not enough to succeed, your friends must fail."