Legacy systems can certainly be insecure (Look up some of the original viruses or famous Windows bugs for good examples) and they can be fantastically secure (banking, electrical grid, nuclear launch codes to use obvious examples). (politics.stackexchange.com)
Somewhere along the way, certain molecular changes, including a deletion of 29 base pairs from the original virus, allowed SARS to infect and replicate in human cells. (discovermagazine.com)