It does seem like we are being confronted with an onslaught of new diseases – Ebola, Zika, MRSA, CRE etc.
But seeming and being are two different things. Our notions of probability and prevalence are strongly biased by their availability to our memory. Dying from these diseases is a horrible prospect that triggers a strong emotional reaction which makes them memorable. And things that are memorable seem very likely.
Here’s a graph that compares causes of death in the US for 1900 vs 2010
That huge green block of infectious disease death in 1900 shrank down to a sliver by 2010. That shrinkage accounts for nearly all of the 20th century reduction in death rate. Deaths from infections are memorable precisely because they are rare.
People are bad at judging and responding appropriately to risk. If there’s a shark attack (risk: 1 in 110 million per beach visit), we stay out of the water. If there’s a terrorist attack (US lifetime risk of dying: 1 in 45,000) we elect right-wing politicians who promise to keep us safe. We drive our kids everywhere (663 deaths, 121,000 injuries per year) to protect them from abduction by strangers (65 per year).
There’s nothing wrong with our immune systems. Our brains are a different story.
I think yes, immune systems are generally weaker because the SAD (Standard American Diet) is so terrible. It takes effort to eat the right things, because the wrong foods are all around us. It takes discipline to do the right thing.