In England, the increase in allergies and asthma after the Second World War was so strong that it was referred to as the “allergic epidemic” or “post-industrial epidemic”. Common roots of allergies, asthma and neurodermatitis were suspected, but the triggers were unknown, and it was assumed that viral infections were the cause. A turning point was the famous publication in 1989 by British epidemiologist David Strachan from the University of London (Strachan 1989). His data suggested that infections, which lead to inflammation in the short term, reduced the frequency of allergies in the long term. Strachan examined the occurrence of hay fever, i.e., an allergy to plant pollen, in over 17,000 children who were born during a certain week in March 1954, and were followed up to the age of 23. Of the 16 points of collected data on birth, social status and environment, two factors showed a clear correlation with hay fever: The larger a family, the less likely the children were to have allergies to pollen in later life. In addition, children with older siblings were better protected against hay fever than the first-borns. Strachan’s unadorned publication takes up less than two printed pages, but has since been cited over 6000 times in scientific publications. In it, he speculated that, as a family grows in size, the younger siblings are exposed to infections that they catch from the older children. He was thinking of pathogens that are transmitted through direct contact or dirt, such as the Hepatitis A virus or the parasite Toxoplasma gondii. Contrary to the prevailing opinion at the time, Strachan suspected that the excessive immune reactions in allergies were not triggered by pathogens, but that infections, on the contrary, provided protection against them.