Before man created towns and cities, his health, well‐being and life‐span were only significantly affected by the interplay of biological factors upon his genetically determined make‐up. In other words, although he burned a little wood producing a modicum of carbon monoxide, pathogenic bacteria, viruses and parasites were his deadly enemies. Soon, of course, he invented war which, apart from precipitating violent ‘sterile’ death, also offered a further ripe ground for pathogens among the wounded. But still the environment — land, water, air — were in a state of great natural purity.