Hygiene hypothesis: why south/north geographical differences in prevalence of asthma and sarcoidosis?

Although asthma is multi-factorial and generally worsens during winter, prevalence of asthma tends to be higher in warm regions. By contrast, sarcoidosis, which like asthma results from immunological abnormalities that disturb the lower respiratory tracts and unlike asthma is characterized by T helper (Th) 1 response, occurs predominantly in colder regions. The hygiene hypothesis proposes that infection by intracellular pathogens such as early childhood viruses promotes Th1 immune phenotype and decreases susceptibility to later occurrence of Th2-associated asthma, since Th1 and Th2 cells are mutually inhibitory. As respiratory viral infections are generally more common under colder conditions, it is hypothesized that more respiratory viral infections in colder climates than one's natural environment during early childhood may promote subsequent occurrence of sarcoidosis, while less infections in warmer environments may promote subsequent occurrence of asthma.