Scientists believe that knowing exactly which type of dirt provides the best 'education' for the immune system, could be key to providing new treatments for diseases such as asthma.

Speaking at the launch of the BA Festival of Science today, Professor Peter Openshaw, explains that a lack of exposure to dirt and common viral infections among children could be behind the rise in the levels of asthma.

Professor Openshaw, from Imperial College London, and based at St Mary's Hospital, says: "Although we have seen a dramatic decline in many previously common childhood infections over the past 100 years, we have also seen a considerable rise in the prevalence of diseases such as asthma. The increase in asthma cannot be blamed purely on changes in genetic risk, so must be down to environmental factors."

Scientists have called this the 'hygiene' hypothesis, with a lack of exposure to viruses and other environmental factors meaning children are not able to build up resistance, and can become more susceptible to disease later in life. They also believe having many older siblings, attending day care at an early age, or growing up on a farm can help in promoting resistance to disease.

Studies have shown that most common colds can help protect against wheezing in later childhood, and other childhood infections such as chickenpox also provide a level of protection.

Professor Openshaw adds: "The challenge now is to find ways of reproducing the protective effects of early childhood infections, while reducing the burden of actually getting these infectious diseases. Knowing exactly which 'dirt' provides the best education for the immune system, and how to mimic its affects in a cleaner environment, could be the key to reducing the rise in the prevalence of asthma and related diseases."

Professor Openshaw is a respiratory medicine researcher, looking at immunological responses to diseases such as asthma, the common cold and other lung diseases.