Bacteriophages have been found to be effective in the elimination of specific pathogens from meat, milk and derived products. Bacteriophages, viruses that infect bacteria, occur in nature and have a broad range of habitats. Their effectiveness depends on their quantity and the condition of the food. A Europe-wide study found that bacteriophages can dramatically reduce salmonella and campylobacter contamination of birds entering processing plants. It is uncertain however whether bacteriophages can protect against bacteria when food is contaminated more than once.