Some of the worst diseases in the world are caused by viruses. SARS, AIDS, and Avian Bird flu are all caused by these strange pathogens that constantly mutate and change to better attack us. Viruses cause about 75% of all food poisoning. This video examines what virsus are, how they cause disease, and how we can keep them out of the food chain. It discusses prions, small molecules that make subtle changes to the way cells function, and how they caused the Mad Cow disease outbreak in the UK in the 1990s.