I worked at the Anaheim Theme Park, 1990-2001. There was a group of illegals that fraudulently obtained employment, and it really hit the fan, when it came to light. All Cast Members (employees) had to re validate their credentials, and those docs were scrutinized with extreme care.
On the other hand, instances of illness was quite common, due to the amount of people visiting, that had been on the other side of the world, literally, within the past 24 hours, bringing new types of germs, with them. Fortunately, I worked in an office, so was not as exposed, but I still found myself getting some ailment, every six weeks, or so.
My doctor told me that, in that environment, I should be sanitizing, or washing with soap, and hot water, every 30 minutes, or so. I thought it was a bit extreme, but guess what, I only got sick about once a year, from that point, on.
Sometimes the best way to stay healthy is also the simplest; plain old soap and hot water. Antibiotics are useful, but in moderation. Teachers go through the same thing on a much reduced scale of risk.