The precise pattern of infection depends on the specific component of the immune system that is affected. Most primary immunodeficiencies (PIDs) are caused by defects in single genes and are hence heritable. Others may represent the consequence of an interaction between the genetic phenotype and an environtmental influence, like viral infections. Primary immunodeficiencies are rare and based on information from national registers; these diseases are estimated to occur between 1 in 2,000 to 1 in 10,000 live births. In contrast, secondary immunodeficiencies are more commonly seen in clinical practice.