Ransomware .cm99v files found (malware)

My MacBook has .cm99v files and text files with a name including the test "How To Decrypt". I have not tried to open any of them. How does ransomware work? Is there a hidden application on my computer that will continue to infect files? Am I being attacked from the internet? Is there the potential that files without the .cm99v suffix are infected? I can reformat my HDD and install a fresh system, but want to know if I can save a few files first. Thank you.