The chapter offers a detailed examination of the intricate dynamics of parasitism in agricultural animals, emphasizing its significant impact on animal well-being, productivity, and farming economics. It begins by providing a precise definition of parasitism and classifying the main types of parasites affecting livestock, which include helminths, and ectoparasites. It then explores the life cycles of these parasites, explaining how they adapt to and exploit their host organisms. In-depth discussions of key parasitic diseases, such as helminthiasis, and external infestations are caused by fleas, mites, and lice. These are included to illustrate the diverse impacts on various livestock species. The global cattle population, approximately 1.49 billion, is vulnerable to various external parasites, including insects, lice, mites, and ticks, which can significantly affect their health and productivity. Internal parasites, or endoparasites, such as tapeworms and flukes, inhabit the digestive or respiratory systems of their hosts, posing severe health risks. Helminths, particularly gastrointestinal nematodes, present major challenges to animal health and productivity in grazing livestock systems. Parasitic lungworms, including Dictyocaulus, Muellerius, and Metastrongylus, commonly infect ruminant animals and cause conditions such as verminous bronchitis. Trematodes, including liver flukes like Fasciola hepatica and Fascioloides magna, and the lancet liver fluke, Dicrocoelium dendriticum, significantly impact the health of livestock. Additionally, tapeworms such as Moniezia benedeni and zoonotic cestodes like those in the genus Taenia, contribute to severe illnesses in livestock and humans. Effective management and preventive measures are essential to mitigate the impact of these parasites on livestock health and productivity. By providing a comprehensive review of existing research and case studies, this section aims to equip veterinarians, farmers, and researchers with a profound understanding of parasitism in farm animals. Several groups of fungi also show parasitism and bring bacterial infections. The immunocompromised livestock are often killed by the heavy infections of various infectious fungi such as Candida, Aspergillus, Malassezia, and so on. The objective is to empower stakeholders with the knowledge to effectively manage and mitigate parasitic infections, ultimately leading to improved animal health and farm productivity.