Sharyn Clough of Oregon State University says the link between increased hygiene and sanitation and higher rates of asthma, allergies and autoimmune disorders is known as the "hygiene hypothesis" and the link is well-documented.
"Girls tend to be dressed more in clothing that is not supposed to get dirty, girls tend to play indoors more than boys, and girl's playtime is more often supervised by parents," Clough says in a statement.
"There is a significant difference in the types and amounts of germs that girls and boys are exposed to, and this might explain some of the health differences we find between women and men."
Clough suggests looking at old studies to look for the gender differences.
"The hygiene hypothesis is well-supported, but what I am hoping is that the epidemiologists and clinicians go back and examine their data through the lens of gender," Clough says.
The hygiene hypothesis links the recent rise in incidence of asthma, allergies and autoimmune disorders such as Crohn's disease and rheumatoid arthritis, with particular geographical and environmental locations, in particular urban, industrialized nations.
For example, many studies note that as countries become more industrial and urban, rates of these diseases rise -- Crohn's disease is on the rise in India as sanitation improves and industrialization increases.