The present invention relates generally to chemical agents useful in the treatment and prophylaxis of infection by pathogenic or potentially pathogenic entities, or entities capable of opportunistic infection in mammals, including humans and primates, non-mammalian animals and avian species. More particularly, the present invention provides a chemical agent of the macrocyclic diterpene family obtainable from a member of the Euphorbiaceae family of plants or botanical or horticultural relatives thereof or derivatives or chemical analogues or chemically synthetic forms of the agents for use in the treatment or prophylaxis of infection by pathogenic entities in mammalian, animal and avian subjects. The present invention further contemplates a method for the prophylaxis and/or treatment in mammalian, animal or avian subjects of infection or potential infection by pathogenic entities by the topical or systemic administration of a macrocyclic diterpene obtainable from a member of the Euphorbiaceae family of plants or their botanical or horticultural derivatives or a derivative, chemical analogue or chemically synthetic form of the agent. The chemical agent of the present invention may be in the form of a purified compound, mixture of compounds, a precursor form of one or more of the compounds capable of chemical transformation into a therapeutically active agent or in the form of a chemical fraction, sub-fraction, preparation or extract of the plant.