The invention refers to an isolated extract of walnuts of high efficiency and stability over time useful for the manufacture of a medicament for the treatment of viral, fungal and bacterial diseases, which is characterized in that it is obtained from a process comprising: i) Collecting unripe walnut fruits as raw material; ii) Preparing the raw material for the extraction; iii) Freezing the raw material prepared in the previous step; iv) Drying; v) Extracting in a time lower than 10 minutes; vi) Filtration; and vii) Final packaging. The invention also refers to its use for manufacturing a medicament for the treatment of bacterial, fungal and viral diseases and to a composition comprising it.