"Some biologists have thought very carefully about certain issues," "These selected species will need a series of modifications to at least be able to adapt to the human microbiological environment."
Cloned from a sterile environment, extraterrestrial species would carry no pathogens from their native planets and be harmless to humans. However, among these species, some thrive in oxygen while others would be poisoned to death by it. Some might fall ill and even die from just a slight exposure to human bacteria.
Living on the same planet, with frequent coming and going, getting sick all the time would be too troublesome.
Even with the strictest control over microbes, there are bound to be occasional oversights. Humanity and the Half-sphere people have cooperated for many years, and in the last biochemical test, human bacteria and viruses had already appeared in the living areas of the Half-sphere people.
