Web MD states exercise and/or active lifestyles boosts the immune system. So I guess that's reason number 1.
Reason number 2 could be the lack of human contact. The most effective way to spread germs is by human contact, like by a handshake. Not much of that on the mound. One supposes a virus could be transported via the baseball, but there are dozens of new balls used in a game.
The Mayo Clinic says:
"...Experiments with specific cold and flu germs have shown potential survival times ranging from a few minutes to 48 hours or more. How long such germs remain capable of infecting you in day-to-day life is harder to say. Researchers have repeatedly found that cold and flu germs generally remain active longer on stainless steel, plastic and similar hard surfaces than on fabric and other soft surfaces (like baseballs?).
On any surface, though, flu viruses seem to live longer than cold viruses do. Other factors, such as the amount of virus deposited on a surface and the temperature and humidity of the environment, also have effects on how long cold and flu germs stay active outside the body."
So that poses the next question: what about natural fertilzers used on the turf? :shrug:
LINK:
http://www.mayoclinic.com/health/infectious-disease/AN01238Pretty obvious I'm bored, isn't it...