PPIA and infection: To investigate the susceptibility of CypA+ mice to infection with influenza A virus, the 50% median lethal dose (MLD50) was determined for each genotype by challenge with a serial dilution series of A/WSN/1933 (H1N1) virus at a dose of 1000, 2000, 4000, or 8000 plaque forming units (PFUs) (n = 5 per dilution, 20 mice per group).