Probably just shaking a lot of hands.

If it survives for 12 hours on surfaces, you only need 1 person to have poor hygiene to expose a LOT of people. And "normal" hygiene levels pass infection around plenty. I wash my hands when I go to the bathroom or before I prepare food. The rest of the time I'm just passing whatever I touch to whatever else I touch. I've given up on not touching my face. I'm sure I'm not the worst in these areas.

As I understand it, you'd have this sort of infection rate with the seasonal flu if there wasn't some level of herd immunity, and you certainly have roughly this level of infection with the common cold - "Con crud" is a thing - it's just not much remarked upon as unexpected.