The spectrum of genetic susceptibilities predisposing individuals to infectious diseases is summarized. Different situations may be distinguished according to the number of genes with an additive impact on genetic susceptibility (in green) or resistance (in red), the marginal effect of each of these genes, and the number of pathogens to which the individual is susceptible. Six textbook examples are shown: SCID-associated infections (a unique gene with complete penetrance conferring predisposition to a large spectrum of infectious agents); HSE (a single gene with high penetrance conferring predisposition to a single infectious agent); malaria caused by P. vivax (a single gene with high penetrance conferring resistance to a single infectious agent); severe malaria caused by P. falciparum (a small number of genes with HbS conferring resistance to the disease); leprosy (a small number of genes with intermediate penetrance conferring predisposition to a single infectious agent); diseases in which HLA alleles have been shown to play a role (HLA-associated infections). Examples of common infectious diseases favored by multiple predisposing alleles in a given individual (truly multigenic inheritance) may be revealed by future GWA studies.