Let's not forget that the reason people get sick is because they are infected. Increasing population density will clearly lead to more transmissions, especially in cities.
Actually, TFA makes the case that people living in developed societies are not infected early enough in life, which makes their immune systems vulnerable in the long run.
@verroq is correct though, but that is a different subject than the one of the article.
(Infectious) diseases generally spread more easily in higher population density. That's basic epidemiology, more contacts = opportunities, but it's of course more complicated than one single statement, disease spread models need a lot more variables than just population density to make useful predictions.
Well, it used to be that infant mortality was a lot bigger, some of which was probably infants dying to things they were exposed to. So we can't just ignore survivorship bias in considering these things.
In general, we're probably better off with controlled, safe exposures (e.g. vaccines) than uncontrolled exposure and simply hoping we'll survive anything we catch.
