Many parents and physicians are observing a worrying trend: more and more children are suffering from food allergies, eczema, asthma, hay fever and other allergic maladies. Why are allergies becoming more frequent when most other childhood disorders are decreasing and our medical treatment options keep improving?

The “hygiene hypothesis” claims that lack of stimulation of our immune systems with bacteria or viruses could lead to higher rates of allergies and asthma. While this approach can not completely explain the rise in allergic and autoimmune diseases, it does offer some explanation for the shocking increases.

To understand the hygiene hypothesis, it is important to know how allergies develop. Allergies are caused by unusual immune responses to things we usually encounter and tolerate without problems, such as animal dander, dust or pollen.

The main function of the immune system is to defend us from dangerous bacteria and viruses. When your child comes in contact with these pathogens, the immune system goes to work fighting them off. But in allergic people, the immune system attacks harmless substances.

The first time your child comes into contact with the allergen, the body’s immune system sends out signals to attack the foreign invader. Often, there is no allergic reaction following the first exposure, but the body becomes “sensitized.” The next time the allergen is encountered, the body responds fast – usually within 30 minutes – and the reaction can be mild or severe.

Professor David Strachan developed the hygiene hypothesis in 1989 while trying to explain the increase in allergic diseases since industrialization, and the higher incidence of allergic diseases in more developed countries. He was the first to publish an articledescribing his observation that allergic diseases, such as hay fever and eczema, were less common in children who came from larger families. He concluded that those children may be exposed to more diseases, bacteria and infections through contact with their many siblings, helping to build their immune systems. Children without siblings might not receive the same exposure.

That’s not the only evidence to support a hygiene hypothesis. Studies have also shown that various immunological and autoimmune diseases are much less common in the developing world than the industrialized world. Interestingly, immigrants who come to the West are likely to develop immunological disorders.

Other studies indicate that immune system stimulation begins in the first moments of life. Birth via cesarean section, rather than vaginal birth, has been associated with a higher risk of developing allergies later in life. Exposure to bacteria in the birth canal can stimulate the infant’s immune system early on.

Does this theory mean that vaccines are bad for our kids?
Actually, vaccinations are important to prevent serious consequences (including death) from dangerous childhood diseases. The majority of studies examining the influence on vaccination of development of allergies have not found an increased risk for allergies or asthma in vaccinated children. In fact, some studies have shown exactly the opposite: vaccinations can actually protect against allergies. However, the majority of studies refute this result, and if there is any effect, it’s probably a weak one.
Is it good for kids to be exposed to bacteria and viruses to prevent allergies?

It’s hard to answer that question. There are good and bad bugs. RSV and Rhinovirus (viruses causing the common cold and respiratory infections), for example, are known to increase the risk for development of asthma in the future. However, exposure to hepatitis virus and intestinal parasites – which are serious health threats - were shown to prevent development of allergies.

It probably is not wise to expose children to dangerous pathogens for prevention of allergies, but in my opinion, the evidence supporting a hygiene hypothesis does suggest that we could probably allow ourselves to be a little more relaxed about hygiene. This would include not using antibacterial soaps all the time, and not wiping our children’s hands constantly with disinfecting wipes—since the use of antibacterial cleaning products has also been associated with higher incidence of asthma.

Antibiotics wipe out bugs. By the hygiene hypothesis, does that increase risk of allergies?
The use of antibiotics in the first year of life has been linked to asthma and other allergic diseases. When antibiotics are needed for treatment of an infection caused by bacteria their benefit will outweigh the risk of developing allergies. However, antibiotics should not be overused.
Are pets in the home good or bad?

Studies seem to show that living with pets early in life may actually protective for development of allergies. Dogs seem to be better for allergy prevention than cats. Of course the best preventative outcomes were observed in children who grew up on farms with exposure to cattle and other barn animals. Several researchers concluded that organisms in cattle dust and manure may be the stimuli that their immune systems needed to prevent asthma.

The hygiene hypothesis can only account partly for the increase in allergies. There is also a genetic component which influences why certain people will develop allergies, but others will not despite living under the same circumstances. An approach to examine the allergy epidemic from different angles, epidemiologic and genetic, is only recently starting and produces fascinating results on the effect particular gene-environment interactions might have in the development of allergy. The most important point is that thousands and millions of separate risk factors act in concordance in the development of allergies.


Corinna S. Bowser, MD, is an allergist/immunologist based in Havertown, Pennsylvania.

The American Academy of Allergy, Asthma & Immunology is the largest professional medical association devoted to the research and treatment of allergic disease. The AAAAI offers educational materials for patients online at