Aspergillus spores are incredibly tiny, tough fungal seeds that germinate to form the germ tube and later hyphae. Germination is affected by temperature, humidity and pH among other factors.
About 3-4% of the population, including many asthma sufferers, are allergic to the proteins coating the surface of fungal spores. Levels of spores from most species peak during June-Sep, but aspergillus also peaks in Jan-Feb.
We all inhale hundreds of Aspergillus spores every day, but in healthy people they are cleared by white blood cells that engulf them before they have the chance to germinate. Stopping them from germinating is one potential way of preventing disease.
This picture shows a scanning electron micrograph of germinating Aspergillus fumigatus spores (provided by KM Lord and ND Read). In this picture the spores are clustered in the middle, with the germ tubes radiating outwards.