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Available for over a year

A look at why viruses that do animals no harm are so dangerous to humans. A report on aggressive symbiosis, why viruses such as Aids and Ebola allow their monkey and ape hosts to survive but will kill humans because they post a threat to the viruses' first host. Also in the programme, a chemical cure for memory loss, why looking to the past could give us clues to the future and safe sun worship.