Food allergies and other allergic and atopic disorders are largely diseases of the modern age. They were rarely diagnosed before the 20th century and are far less prevalent in underdeveloped parts of the world where bacterial and parasitic contamination of water and food supplies is common and access to vaccination and health care is poor.

The epidemic rise in allergic disease and asthma in the United States and throughout the Western world has coincided with a decline in infectious disease, due to improved public health, vaccination programs and better hygiene.

Is there a link between the risk in allergic disease and asthma and decline in infectious disease? According to the hygiene hypothesis, the decrease in exposure to microorganisms in the US and other prosperous nations is directly responsible for the dramatic increase in disorders that involve exaggerated immune responses to foreign antigens.

The theory that exposure to certain infections and allergens early in life or in utero protects against allergies and asthma has been bolstered by numerous epidemiologic and animal studies and even some clinical trials since it was first proposed by epidemiologist David P. Strachan of St. George’s University London.

Strachan showed that having more siblings was associated with a lower incidence of hay fever, and he postulated that exposure to infections from contact with older siblings protected against allergic disease.

But 25 years later, the hygiene hypothesis remains just that, experts say.

While there is strong evidence supporting the idea that increasingly clean environments have led to the rise in allergic and atopic disorders, other evidence suggests the hygiene hypothesis may only be part of the story, said Jonathan I. Silverberg, who is an assistant professor of dermatology at the Northwestern University Feinberg School of Medicine in Chicago…

Silverberg’s own recent research showed a lower prevalence of asthma, eczema, hay fever and food allergies in US-dwelling children born in other countries compared to children born in the United States. Foreign-born American children were significantly less likely to have developed one or more of these diseases than US-born children…

However, the protection associated with foreign birth was not evident in children who had lived in the United States for 10 years or more…

Silverberg and colleagues noted that the findings were consistent with the hygiene hypothesis, but they also suggested that protection may not be life-long and that subsequent exposure to allergens and other environmental factors may trigger atopic disease later in life…

He added that dietary changes, increased obesity, and environmental pollutants could all play a part in the increased risk associated with living in the US…

One consistent finding in the hygiene hypothesis research is that growing up on a farm is protective against allergic disease and asthma.

“This has been shown all over the world,” epidemiologist Neil Pearce said.

The more you hang out with animals the healthier you are likely to be.

I have no idea if that’s provable or not. Just seems to be another possibility. RTFA for all the details, considerations in the whole article.