There's not really any proof that hygiene hypothesis leads to better outcomes. Yes, the immune system generally only strengthens when it's exposed to pathogens,

There's not really any proof that hygiene hypothesis leads to better outcomes. Yes, the immune system generally only strengthens when it's exposed to pathogens, but being exposed to lots of pathogens also damages your body. And controlled exposure to pathogens (ie, vaccines) can trigger an immune response without injury. So, we're likely trading weaker immune systems for millions fewer deaths. Look at the mortality rates due to infectious disease in countries with poor sanitation.