Melvyn Bragg and guests discuss the relationship between parasites and hosts, where one species lives on or in another to the benefit of the parasite but at a cost to the host, potentially leading to disease or death of the host. Typical examples are mistletoe and trees, hookworms and vertebrates, cuckoos and other birds. In many cases the parasite species do so well in or on a particular host that they reproduce much faster and can adapt to changes more efficiently, and it is thought that almost half of all animal species have a parasitic stage in their lifetime. What techniques do hosts have to counter the parasites, and what impact do parasites have on the evolution of their hosts? With Steve Jones Emeritus Professor of Genetics at University College, London Wendy Gibson Professor of Protozoology at the University of Bristol and Kayla King Associate Professor in the Department of Zoology at the University of Oxford Producer: Simon Tillotson.