"The (SARS Corona) virus could mutate and escape the body's immune system. Was it really eliminated? Yes, because it is no longer infecting humans. Then again, it could be lurking in its original host or reservoir. It could return if naive humans (not immuned) come into contact with the virus and if the infected person replicates the virus efficiently and subsequently infects others. Only those who were infected and recovered would have a strong immune defense against the virus. This is because immunity has a long-term memory that could recognize the original SARS virus."