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Disease caused by gastrointestinal nematodes may be acute in onset, with clinical disease, including some mortality, affecting >10% of a flock. However, these parasites also cause sub-clinical disease, with reduced growth rates, milk and wool production and body condition which result in significant financial loss to the industry. The pathogenicity and severity of the various GI parasites are outlined in this section, together with detail on the development of immunity to internal parasites and how this is an important factor in the reduction in our reliance on anthelmintics.