Pathogenic bacteria and fungi have developed the means of survival in animal tissue. They may invade the gastro-intestinal tract (Escherichia, Shigella and Salmonella), the lung (Pseudomonas, Bordatella, Streptococcus and Corynebacterium), skin (Staphylococcus) or the urinary tract (Escherichia and Pseudomonas). Such bacteria may colonise wounds (Vibrio and Staphylococcus) and be responsible for septicaemia (Yersinia and Bacillus). Some bacteria survive for long periods of time in intracellular organelles, for instance Mycobacterium. (see table). Because of this continual risk of bacterial and fungal invasion, animals have developed a number of lines of defence based on immunological strategies, the complement system, the production of iron–siderophore binding proteins and the general “withdrawal”of iro