Recent discoveries in innate immunity together with improvements in the analysis of the human genome have led to the identification of factors that make certain individuals more susceptible to infections than other. We know understand why herpes simplex virus I, a virus with a minor burden in most individuals, is responsible for devastating encephalitis in children unable to detect primo-infection of neural cells. A growing number of patients are treated with immunosuppressive drugs in the field of oncology, organ transplantation and immunology. In such patients, opportunistic infections such invasive aspergillosis are clearly associated with genetic polymorphisms. Medical immune suppression represents a possible model for personalized approaches, in which infections could be prevented by individualized prophylactic strategies based on genetic testing.