In Jan 2019 Professor Graham Rook recorded a podcast on current thinking about this issue. In it he traces growing awareness of the fundamental importance of the microbial world in which we live, and which lives within us (the human microbiome), to our health and how altered interaction with our microbial world is underlying a whole range of diseases, not just immunological diseases, which have rapidly increased in recent years.
In talking about how microbes interact with our immune system, he likens the immune system to a computer programme i.e. we are borne with a fully functioning immune system but it lacks data. Programming by exposure to microbes is vital to ensure that it reacts to things which are potentially harmful, but tolerates those which need to be tolerated. Talking about the types of organisms we need – he is clear that these are not the common “crowd infections” of childhood which hygiene and vaccination measures were developed to control – as the 1989 hygiene hypothesis proposed. He says “These appeared much too late in our evolutionary history to have evolved an essential role in the development of human immune systems. The organisms that we require are the microbiota of our mothers, and organisms from the natural environment”. Professor Rook calls this concept the “Old Friends mechanism”.
Answering the question “What’s gone wrong” Professor Rook is again very clear that it’s not “too much hygiene and cleanliness”. “Promoting this simple message is just wrong and has major health implications”. He says problem is probably the combined effect of a whole range of lifestyle factors such as the preference for C-section childbirth, less breast-feeding, smaller family sizes, altered diet, less time outdoors in the natural environment, - he says “One of the most important factors is likely to turn out to be the widespread use of antibiotics and its adverse effects on the microbiome and its diversity”
The podcast can be found at: https://soundcloud.com/adoseofnaturepodcast/a-dose-of-nature-podcast-episode-5-prof-graham-rook-and-the-old-friends-hypothesis