This invention relates to a method for preparing compositions for preventing or treating microbial infections, compositions suitable for use in such treatments and methods for treatment or prevention of infections. The compositions comprise iodide and a source of hydrogen peroxide. One such composition finds particular use in treating mastitis in ruminants. The composition is administered into the udder of an animal as a highly effective treatment for mastitis, or as a prophylactic therapy, by means of an intra-mammary infusion. The milk produced by the animal, during treatment using the composition and method of the invention, is free of residues, such as antibiotics, antimicrobial agents or antimicrobial proteins, which could affect its suitability for drinking or in the production of milk products, such as cheese or yoghurt. The compositions and methods are also useful in treating and preventing lung infections； and infections in bums and wounds； and other infections caused by biofilms. The compositions may also be used on medical devices to prevent infection.
