Backgrounds: Infectious diseases are one of the main leading causes of morbidity and mortality worldwide. Therefore, diagnosis and treatment of infectious diseases are very important. Infection affects some blood indices that could be used as diagnostic markers. Therefore, this study aimed to compare blood parameters in four important and prevalent infectious diseases. Materials & Methods: In this study, 202 out of 892 patients with a final diagnosis of UTI (urinary tract infection), septicemia, pneumonia, or diabetic foot infection were evaluated; they were hospitalized in Rasoul Akram hospital in Iran from August 2017 to February 2018. Patients' demographic and laboratory data, such as RDW (red cell distribution width), PDW (platelet distribution width), RBC (red blood cell), CRP (C-reactive protein), ESR (erythrocyte sedimentation rate), and, WBC (white blood cells), were evaluated. Findings: This study results showed that mortality rate in sepsis cases was higher than in other cases (42.1%). Changes in blood parameters such as RDW, PDW, and EDR levels as well as monocyte, basophil, and eosinophil counts were significant among patients with different infectious diseases, while there was no significant difference in terms of changes in some blood parameters, such as WBC, neutrophil, and lymphocyte counts and CRP level between patients with different infectious diseases. For statistical analysis, one-way ANOVA and LSD post hoc tests were used. Conclusion:According to this study results, it was found that the range of blood parameters varies in different types of infectious diseases. Therefore, the physician could employ routine blood parameters along with other diagnostic factors to more accurately diagnose the type of infection and prescribe more appropriate antibiotics.