A hard one but you did do your Sarria Camino. I think it is travelling elsewhere that can do this .. we have our own immunities, go to a different country with different strains and we can get hit.
Also, since the smoking ban on aeroplanes they have become a perfect place to catch a new bug - when smoking was allowed they recycled the air through the cleaners about every 30 seconds, so constantly really, but the system uses fuel. When the ban came in, to save fuel and therefore money, they dropped it to maximum once every three minutes, on some aeroplanes much less, so one sits in a container of shared bugs - which suggests, the bug kicking in some 72 hours later, that you probably caught your bug on your flight - oh, and probably shared it on your flight home