Over the last 30 years evidence has been accumulating that many people who develop severe asthma also show evidence of a fungus (mold) in their lungs called Aspergillus.
Many readers will be familiar with aspergillus as the fungal mold that can cause severe invasive infections in mostly immunocompromised people. High Risk, low immune; include children, elderly, patients with illness', and some medicines that can lower the immune system. But realize that Aspergillus at high levels can also effect persons who are completely healthy.
Chronic infections caused by Aspergillus are CPA which is "Chronic Pulmonary Aspergillosis", shows up in many patients with scarred airways or lung tissue due to previous infections. Once scarred the airways tend to lose their 'cleaning system' for that area, allowing infections to settle in.
Apart from these exceptions people who have healthy immune systems are pretty safe from infection by Aspergillus- except for asthma and Allergic Bronchopulmonary Aspergillosis (ABPA).
ABPA is a well described condition effecting some immunocompetant people who develop a chronic non-invasive infection in their lungs. People vulnerable to this include asthma and cystic fibrosis.
Asthmatics; around 25-50% have fungal sensitivity (SAFS), equating to about 1-3 million persons in the US. Some doctors are already treating SAFS with an anti-fungal medication with good success.
Good Indoor Air Quality can reduce and eliminate these aspergillus spores from the indoor environment, where for example children now spend well over 90% of their time.