As humanity left for the stars, bacteria hitched a ride with us. They were on us, in us and around us - as much part of us as our own cells. Viruses, on the other hand, due to careful screening programs, quarantines and selective breeding programs of the few animals that took to the stars, were confined to the gravity well of Earth. Without hosts, they could not follow us. We had left our ancient enemy behind.
Within two generations, the space born growing up on the O’Neill colony cylinders - even those who lived at 0.9 to 1g - realised that earth was going to be a forbidden planet for them. They had no defences against the viruses that continued to permeate the planet. A visit to earth required them to live in an isolation suit and sterile quarters - limiting contact with the earthers and fauna.