I think there's confusion. Chicks don't always eat and drink by themselves. Chicks often arrive and MUST have their beaks dipped in warm sugar water, and be put under lamps. In fact, they are shipped with this as an instruction. If they didn't get that, they may not eat and drink by themselves and can die. They are not automatically "sick". If they ate and drank on arrival, then they were not likely sick. It takes several days of exposure to a bacteria or virus for them to get sick and they haven't even been alive that long, and in an NPIP flock it would be very unlikely. Unless the shipping was longer than 2-3 days, they have enough of the yolk to sustain them. Chicks not doing well is not usually a "sickness". It's usually from error in how the chicks were acclimated or they didn't get sugar water and set under the correct temp lights. These chicks come from NPIP flocks, so are monitored for everything. The parents of the chicks are kept in very good condition to producee the highest number of strong chicks, or they are culled. Other than shipping stress or unusually cold conditions while being shipped, it's basically considered arrival issues.