If you're stressed out all week and you pick up an infection then your immune system might not settle into fight mode until the weekend, because you can't afford to be down for the count when what you really need right now is to run from the bear, except it's not a bear, it's just the grind.

If there are more potential infections out there than there were before (also, greater population sizes to communicate them) that's a higher concentration of weekends lost to sickness.

That's gonna have an impact on how often you go to a friend's house... squared, because it's both you and that friend. Like the network effect's evil twin.