It is well known that when a group of people are introduced to a new virus even one as simple as the common cold that their immune system will not be able to handle it and it will make them much sicker than people who are used to it and built up an immunity. It happened to the natives of Americas and the explorers. Scientists fear that Earth will be introduced to an alien Virus 
 
So my question is would it not seem possible that when you have Superman, Green Lantern, and so on planet hopping all the time would they not spread disease and viruses between planets?  
 
Maybe Superman's Kryptonian suit kills all viruses and The GL Ring as well. Or the JLA has a immunization chamber. 
 
What do you think?