This study evaluated the potential benefit of providing dietary supplements to broiler chickens infected by pathogenic Escherichia coli with Allium sativum (A. sativum). Eighty-one-day-old Hubbard chicks were allocated into four groups (20 for each). The first was a control group (no supplement and not infected). The second received supplements of A. sativum 1 g/kg feed from day 1 to the end and was not infected. The third group was experimentally infected orally at 7 days old with E. coli O78 107 CFU intra-crop and received no supplement. The last group was supplemented with A. sativum and infected with the E. coli O78 as described previously. Chickens infected by E. coli had significantly low lymphocytes and a significant rise in heterophils and monocytes with non-significant changes in total leukocytic count (TLC) compared with the control. The fourth group had non-significant alterations in TLC with significantly low monocytes and heterophils and a significant rise in lymphocytes compared with the third group. The biochemical values of hepatic enzymes, renal, and immunological indicators had severe argumentative alterations related to E. coli infection. Dietary supplementation with A. sativum recovers the chickens’ immune indicators in non-infected groups and has an immune-stimulatory effect on E. coli infection. In conclusion, A. sativum mitigates hematological, biochemical, and antioxidant alterations induced by experimentally E. coli infection in broiler chickens.