We don’t know it hasn’t infected humans before. We know this particular strain hasn’t infected anyone that was tested since viruses were isolated and classified. It is possible this virus has been infecting populations in China for years but they have managed to cover it up or it may not have been as severe. It's called a novel, or new, corona virus because scientists haven't seen it in humans. I'm sure they have ways of telling. I think it has something to do with what viruses leave behind in the human body.