Introduction
Science has its fashions. Suddenly the leading science joumals are full of articles about a specific topical research area. Sometimes, this wave of popularity follows a technological break-through which permits asking ques tions that were previously impossible to tackle or at least very hard to address experimentally. At other occasions, this cumulating of top-level research reports is the conse- quence of large international research efforts where grant agencies provided large amounts of money, which
attracted many scientists to the field. In still other situa- tions, the scientific community realizes that a certain field of scientific inquiry has simply been overlooked or neglected and the view offered by the new insights is exiting theoretical interest and promising practical appli- cations. The human microbiome is currently such a fashionable field. Novel DNA sequencing techniques combined with new bioinformatic tools and the general progress of -omics technologies offer the methods: major research grants on both sides of the Atlantic pro- vided the money and the field has been an eye-opener for microbiologists which might be compared with the time of Leeuwenhoek when microbes in our mouth were first seen in the microscope and the time of Koch when the first isolated bacterial colonies were seen by the naked eye and linked to human disease. We perceive the human microbiome metagenome as our second human genome, as a source of human genetic variability (Schloissnig et al., 2013) and as a factor influencing human health (Clemente et al., 2012). The human gut microbiome has been associated with health issues of central importance such as obesity (Turnbaugh et al, 2006), healthy ageing (Claesson et al, 2012) and most recently cancer (Arthur et al., 2012), to quote only the most prominent fields. Probiotic bacteria have also been fashionable for a while (Thomas et al, 2010) and were judged to have a scientific basis (Neish, 2009), but scientific reports aroused less attention than gut microbiota research. Probiotics carry in their definition as 'live health-promoting bacteria' the concept that microbes can influence our health. But what actually is health? If you want to boost health, you must
know what it is and how to measure it
Health: ask the experts
At school we heard of Socrates who asked people who
are supposed to be experts and to get an answer from a
dialogue with them. Therefore I first went to health authori
ties like medical doctors and their authoritative textbooks
that guided generations of medical students like Harri
son's Principles of Internal Medicine (Longo et al, 2011).
In the 18th edition you find ample material on pathogens,
even a chapter on the human microbiome (Gordon and
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