Specifically, parasites in Per1/2-null TRF mice had the highest amplitudes (mean ± s.e.m.: 0.85 ± 0.08) followed by WT ad libitum infections (0.75 ± 0.03), and then WT TRF infections (0.59 ± 0.07), with Per1/2-null ad libitum infections (0.41 ± 0.09) exhibiting approximately half the amplitude of parasites in hosts with feeding rhythms. This evidence concerns the gene PER1 and infection.