Asthma is an overactive immune response to things entering the respiratory system that are not necessarily pathogens. It is one of many autoimmune diseases like Crohn's, Alzheimer's, Arthritis, Multiple-sclerosis (MS) and Arteriosclerosis. Autoimmune diseases don't exist where parasitic worms still prevail.

I recently listened to an iTunes U Yale University lecture series on the Principles of Evolution, Ecology and Behavior by Professor Stephen C. Stearn that is also available on YouTube. It included a current theory that postulates that parasitic worms suppress the immune system to survive, and the evolution of humans has been slow to adapt to sanitation, so the absence of parasitic worms causes humans to produce cytokines at a rate that cannot be filtered from the system fast enough to avoid an excess, which attacks the body in various ways that result in the aforesaid diseases.

Experiments conducted in Brazil in 2002 indicated the theory is correct.  Scientists in Argentina injected MS patients with a relatively benign parasitic worm eggs and published their results (backup) in 2007 after a 4.6-year follow-up study. The infection arrested the disease. It would appear that parasitic worms control the expression of the Chromosome 5 genes involved in immune response.  The therapy has already proved useful in treating some sufferers of inflammatory bowel disease. The University of Michigan is preparing to conduct a similar experiment using Helminth worm eggs given orally.