Background: Due to their potential for treating a variety of common illnesses, medicinal plants have recently attracted a lot of attention, also other medicinal assertions are now supported by a wealth of scientific evidence, so nowadays natural products such as essential oils (EOs) and crude extracts provide boundless opportunities for novel drugs. Objective: This Study was carried out to investigate the antiviral and antibacterial activity of an essential oils product, Deca-Cel®, against Newcastle disease virus (NDV), Lasota strain, as a viral model and multidrug-resistant Escherichia coli and Salmonella Typhimurium as bacterial models. Methodology: The antiviral activity of these EOs was systematically studied in three experimental protocols viz. virucidal, therapeutic and prophylactic assays employing in ovo model. Firstly, toxicity study was estimated for screening the optimal non-toxic concentration of the EOs in the embryonated chicken eggs and then their antiviral efficacy was determined. Embryo survival was observed by candling daily and the survival rates of embryos were recorded on day 4^th post-inoculation (pi). After the end of experiments, survivors were killed by chilling the eggs in a refrigerator for further examination and allantoic fluid from treated eggs was collected for rapid hemagglutination (HA) test to detect NDV. Minimum inhibitory concentration (MIC) values are used to determine the susceptibilities of some bacterial agents to these EOs to evaluate their antibacterial activity. Results: For studying their antiviral activity, it was found that Deca-Cel® oil can completely inhibit NDV growth with a high embryo survival rate reaching 100% with -ve HA activity as a virucidal and prophylactic agent, while 60% embryo survival rate and negative hemagglutination activity had been recorded as a therapeutic agent, comparing to virus control which showed 100% embryo mortality rate within 48:72hrs pi with strong positive HA activity. Regarding the antibacterial activity, it was found that MIC values of these EOs were 0.4 µL/ml and 0.2 µL/ml against Salmonella Typhimurium and E. coli respectively, which means that these EOs can be used as a good alternative to antibiotics that recently showing resistance. Conclusion: The current findings have demonstrated that these EOs have promising antiviral and antibacterial properties against many avian pathogens.