The ancient Egyptians noted it three and a half thousand years ago. Early 19th century physicians recorded the first accurate descriptions of it. But it's only in the 20th century - and particularly in the latter part of it - that asthma has gone on the rampage. In most of the world's developed countries, it is now a significant public health issue. About one in five Australian children display symptoms of asthma - a figure which has doubled in the last two decades. Whichever way you look at it, it's a lot of people living with the possibility that, at any moment, a lead weight could land on their chest and literally squeeze the air from their lungs.The fact is that being born in Australia gives you greater risk of developing asthma. The question is why?