I wrote a while back about the hygiene hypothesis and how exposing a baby with the asthma gene to certain bacteria while his immune system is developing in the first few weeks of life may prevent asthma. Likewise, lack of exposure may actually cause asthma.
In my recent search of the Internet I found this explains that bacteria present in house dust mites may determine if a child in that house develops asthma. They explain that recent research shows household dust mite actually holds "diverse" amounts of bacteria.
Likewise, "bacterial populations are greatly impacted by the presence of dogs and cats and whether or not children attend day care." Of course we have hypothesised that exposure to day care centers and dogs and cats diminishes the chances of a child getting asthma.
The article also notes that "Additionally, dust samples collected from homes of infants, with or without pets and varying day care attendance, showed differences in dust bacteria were linked with asthma development in children."
Slowly but surely the pieces of the asthma puzzle are coming together.