I definitely think people should have some level of concern. Our PWD has come down with something at the specialty, and my mom suspects dog flu. She's been lethargic and vomiting since Saturday. It could be something else, but they got there Thursday then she came down with it Saturday, so the timing fits some sort of flu.
Whether people should be very concerned or not... Everything I have read said it's unpleasant for the dog (much like flu is for us) but for most it's not life threatening. I believe if people are very concerned, there is a vaccine that can be given.