The transmission of illnesses would be greatly
reduced if companies gave decent sick time benefits.
Is transmission of illness a bad thing? By getting constantly a little bit sick it builds you up. And there's that whole thing about allergies from enthusiastic immune systems, etc.
Also, often diseases are contageous before you are aware of them. I think chicken pox is like this, maybe common virii also.
General immunity only build up until about 20 years old (end of adolescence, basically), sorry, but can't remember the source. Afterwards, exposure to disease does not contribute anything to general immunity. Specific immunity to colds is impossible, as the virus mutates several times per year (you never get the same cold twice). Also, frequent colds tend to lower general immunity, opening the door for secondary infections.
Also, often diseases are contageous before you are aware of them. I think chicken pox is like this, maybe common virii also.