The current study aims to investigate the causes of high mortality rate in suckling calves due to bacterial and viral infection in relation to locality, seasons and age as well as through a high light on the histopathological effect of isolated organisms on tissues of dead suckling calves using both light and electron microscope. This work was performed on a total number of 144(25.1%) dead calves from total reported diseased 573 suckling calves. Samples collected from Al Sharquia, Al Behaira and Al Giza Governorates of Egypt during May 2016 to February, 2017. The mortality rate recorded in Al Sharquia, Al Behaira and Al Giza in percentage of 31.2%, 22.4% and 18.6 % respectively. Mortality rate was more prevalence in winter 69.4% than summer 30.6%. Enteritis cases were reported in 13.2 %, Pneumonia represented 21.5% while pneumoentritis was 65.3%. Isolated bacteria and viruses from out of 144 suckling dead calves suffering from enteritis were Escherichia coli (E. coli) 2.08 %, Salmonella typhimurium (S. typhimurium) 2.7% mixed infection of BVDV with E. coli 4.1%, and mixed of Rota virus with S. typhimurium in 4.1 % of cases. In Pneumonic Calves, the most prevalence isolates were Pasteurella multocida (P. multocida) 6.9% followed by Bovine Viral Diarrhea virus (BVDV), as a single infection, 6.2%, meanwhile mixed infection of both microorganesms were 15.2%. On the other hand, mixed isolates, bacteria and viruses, from 144 suckling dead calves suffering from pnemoentritis wererepresented in 84(58.4%) cases of dead examined calf, Isolated organisms were mixed infection of BVDV with S. typhimurium and E. coli 13.8 %, other cases showed mixed infection of BVD with S. typhimurium 13.8 %, other cases showed mixed of P.multocida with S. typhimurium 13.1 % finally cases of mixed infection of BVD with E.coli and P.multocida 18.7%. Pathological examination of Pneumonic calves revealed marked changes represented by two types of pneumonia which observed as Acute fibrinosuppurative bronchopneumonia (6.9%), and Interstitial pleuropneumonia (21.5%),entric calves revealed Hemorrhagic enteritis (13.2) %. In addition, in pneumoentric calves which represented 58.4%of dead calves andshowed, Fibrino-necrotizing bronchopneumonia, Necrotizing ulcerative enteritis, lymphocytic hepatitis with congestive cirrhosis, Interstitial nephritis, Acute hemorrhagic myocarditis and Splenic lymphoid depletion.