PDCD1 and infectious disease: Similar to other non-infectious diseases, gene polymorphisms have been found to play a noticeable role, which has been confirmed by Single Nucleotide Polymorphism (SNP) analyses on genes such as Cytotoxic T-lymphocyte-associated Protein-4 (CTLA4), Programmed cell Death-1 (PD1) and Endoplasmic Reticulum Aminopeptidase1 (ERAP1) 7–9.