NYC subway alone has almost 2 billion riders annually. Small enclosed spaces where everyone has to breathe the same air, be packed in close to each other, and

NYC subway alone has almost 2 billion riders annually. Small enclosed spaces where everyone has to breathe the same air, be packed in close to each other, and share handles to keep from falling over. LA subway averages 100 million riders annually. One can only imagine that this is likely a huge transmission factor.