There is still a controversial debate as to the potential contribution of the ‘hygiene hypothesis’ to the development of childhood asthma and allergies. Viral infections have for long time been recognized as potent triggers of asthma exacerbations. Whether viral infections also induce the development of new cases of asthma remains unclear, particularly for respiratory syncytial virus (RSV) infections. Other infections have been proposed to have protective effects on the development of asthma and allergies such as Hepatitis A, Helicobacter pylori or Toxoplasma. The available evidence is reviewed in the following paragraphs. To date there is no indication that immunizations of all kinds may exert a negative or positive effect on the incidence of asthma and allergies. In turn, the finding that children in farming environments exposed to a variety of microbial compounds have substantially lower rates of respiratory allergies strongly supports the ‘hygiene hypothesis’.