One can be infected with the influenza virus and transmit it to others without having symptoms. received wisdom is that you are most infectious when the symptoms are least (or not) noticeable.
Think about it, a hundred years of medicinal treatments has caused quicker evolution in viruses. The ones that work better are those that transmit in stealth mode. So they survived to proliferate.

Apparently the human genome reveals that there was a shift in the genome at about the time of the Black Death, or possibly Bubonic Plague. The survivors conferred immunity to their offspring, or in scientific terms evolved.

The warning is that viruses and bacteria evolve faster than humans, and a damn sight faster than medicated humans.

I jab!