The film examines the profound claim that most, if not all, of the degenerative diseases that afflict us can be controlled, or even reversed, by rejecting our present menu of animal-based and processed foods.
Despite the most advanced medical technology in the world, we are sicker than ever by nearly every measure. Cases of diabetes are exploding, especially amongst our younger population. About half of us are taking at least one prescription drug and major medical operations have become routine. Heart disease, cancer and stroke are the country's three leading causes of death, even though billions are spent each year to 'battle' these very conditions. Millions suffer from a host of other degenerative diseases. Could it be there's a single solution to all of these problems?

We are in the grip of an allergy epidemic. 50 years ago one in 30 were affected, but in Britain today it is closer to one in three. Why this should be is one of modern medicine's greatest puzzles. In search of answers, Horizon travels round the globe, from the remotest inhabited island to the polluted centres of California and the UK. We meet sufferers and the scientists who have dedicated their lives trying to answer the mystery of why we are becoming allergic to our world.