It wasn’t until you crowded thousands, or tens of thousands of them, along with their animals, into small dense areas with poor sanitation that disease outbreaks took off. Instead of meeting dozens of new people per year, an urban dweller probably encountered hundreds per day. Diseases that would have affected only a few people at a time as they spread slowly across a continent (or just burned out for lack of new carriers) would now leap from person to person in a flash.
He is talking about the effect of urbanization on disease. But the point is to use this as a metaphor for social media’s effect on our mental life. He says that perhaps diseases of the mind are now spreading quickly. As with urbanization, the trick with social media will be to obtain the benefits and to contain the risks.
Possibly related: in the Peterson/Haidt discussion I referred to the other day, they talk about how the sense of disgust may have evolve to protect people from disease. We tend to feel an instinctive disgust toward groups with customs and manners that differ from our own. If you can overcome this instinct to feel disgust when you are around foreigners, then you can benefit from their ideas and culture. But you increase somewhat your risk of contracting disease. Peterson describes Adolf Hitler as operating on the theory that having Jews or Gypsies in a population was like having rats in a factory. He was so concerned about the disease that might be spread by such creatures that he wanted them eradicated.