Re: Regarding sterile environments and the common cold...

There is a fair amount of study looking at the function of the immune 
system for mice bred and housed in "germ-free" environments. It is 
important to remember that in your intestines there is a considerable 
number of bacteria present of different varieties. So a truly "germ-free" 
person would be lacking those bacteria as well which would cause problems 
with our digestion (many of the things we eat require the aid of the 
bacteria in the gut to be digested).

In germ-free mice it is clear that over time there is some degradation of 
the immune system in the sense that the number of peripheral blood 
mononuclear cells drop and antibody levels drop as well. However, these 
mice are still fully resistant to bacterial and viral pathogens. It is 
true that immunological memory can decline over time but that is true 
whether you are germ-free or not. No studies have been down (that I know 
of) to determine if the rate of decline is any different in germ-free 
animals.

You cannot necessarily extrapolate what happens in the mouse to a human 
but we can compare humans from developed nations versus third-world 
nations. People from countries such as the United States have above-
average health care, are vaccinated for a number of infectious diseases 
and have clean drinking water which effectively limits the number of 
pathogens we encounter in our lifetime. As a result the peripheral blood 
mononuclear cell number and antibody levels are significantly reduced 
compared to people from third-world nations where water is unclean and 
infectious diseases are rampant. People from the United States may require 
vaccination against native diseases when traveling out of the country but 
they are no more susceptible to these diseases than the native population.

It is an important question, however, and one that I am sure the people at 
NASA are very keen to answer with the astronauts aboard the International 
Space Station - although they are not technically in a sterile 
environment.