Abstract: The invention relates to pharmaceutical preparations which are suitable for treating viral infections, including influenza and immunodeficiency diseases and which can also be used in vitro, to inhibit fusion of virus-infected cells with non-infected cells, which preparations contain modified proteins or polypeptides as active substance or substance contributing to the action or as carrier for other substances, which are also active, which modified proteins or polypeptides have acquired an additional net negative charge by derivatisation of their amino groups and/or other basic functional groups with aconitic acid which prevents protonation of basic amino acids and/or other basic functional groups or replaces basic amino acids and/or other basic functional groups by one or more functional groups having a negative charge, and to modified proteins and polypeptides themselves and their preparation and use.