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Abstract
Extract
Animals are protected from infectious diseases by non-specific and specific mechanisms. The non-specific defenses include a number of physical host factors, such as intact epithelium, mucus secretions and cilia of the respiratory system, which act to prevent or impede infection. Occasionally, either because of trauma or adverse effects of the environment on such barriers or because of the invasiveness of a particular pathogen, an infection becomes established and disease develops; host resistance then becomes mainly a function of specific, acquired immunity.