In the 21st century we will rediscover the germ theory of disease: germs not only cause infection as described in standard textbooks but also have a pathogenic role in autoimmunity, atherosclerosis, cancer and even acute psychiatric conditions. In order to reduce morbidity and mortality caused by common organisms we should ensure that exposure is early, often, by the mucosal route and in low dose. Micro-organisms should be delivered daily throughout life by respiratory mucosal spray or enteric coated pill, in precise dose and in a predetermined schedule.