Parasitic infection in human alimentary tract causes a significant change in immune system through its continuous antigens secretion. Statistical analysis showed a significant increase in regulatory T cell percentage in infected patients compared to healthy group (P<0.001). Patients infested with Giardia showed significantly higher CD4+CD25+Foxp3+ cell percentages than those infested with other parasites (P<0.001). In conclusion, natural regulatory T cell frequencies (CD4+CD25+Foxp3+) increase significantly in patients with parasitic diseases compared to healthy controls. The higher levels were associated with mixed infection compared to single infection, and in older than younger patients.