I thought I’d take a bit of time today to discuss how the occurrence of new diseases may have been helped along by humans triumphing over many infectious diseases. This idea is called the Hygiene Hypothesis. The Hygiene Hypothesis suggests that the increasing incidence of autoimmune diseases (like eczema) and allergic diseases (like asthma) in middle and higher-income countries is a result of decreasing incidence of infectious diseases. Basically, becoming cleaner and having fewer infections has resulted in our bodies misdirecting those immune responses, causing us to have allergies and autoimmune disorders. Evidence for the relationship between the decline of infections and the increase in autoimmune disorders and allergic diseases comes from animal models and a number of promising clinical studies.
People living in middle and higher-income countries decreasing their burden of parasitic worms, also known as helminths, is the main concept of the Hygiene Hypothesis. Helminths are a broad range of intestinal parasitic worms, including roundworms and hookworms, common in lower-income areas of the world. People become infected through the fecal-oral route in most instances, so increasing sanitation practices to limit fecal-oral spread of disease results in fewer helminth infections.
According to Yazdanbakhsh, Kremsner, and van Ree (2002), while there is considerably lower prevalence of allergic diseases in lower-income countries, there are also clear differences in the prevalence between rural and urban areas within a country. There is little consistent evidence to suggest that obvious urban risk factors like indoor allergens, pollution, or changes in diet could account for the higher prevalence of allergies in urban populations compared to rural populations, as seen in some countries. However, studies have shown that more childhood infections mean less prevalence of asthma. First-born kids tend to have higher sensitivity to allergens, which is less frequently seen in kids from large families and kids that go to daycare. This suggests a frequent exchange of infection might be beneficial to prevent the development of allergies or eczema.
Researchers have shown that your immune system has two arms, Th1 and Th2. Th1 deals with viral infections, bacterial infections and autoimmune disorders, while Th2 deals with helminth infections and allergies. How does this affect my allergies, you might be asking? Not having enough exposure to bacteria/viruses/helminths when we’re kids may lead to autoimmune disorders and asthma because we’re not properly stimulating and balancing these two arms of the immune system. Readers may have heard about the antibody IgE being implicated in the allergy response, which is true, but do you also know what else stimulates an IgE antibody response? That’s right, the helminths. So not having helminth infections means that those IgE antibodies are just hanging out in our bodies with nothing to do and they can sometimes get confused when exposed to things like dust mites and insects; things that are normally not an issue but remind the immune system of helminths and subsequently cause an immune response.
So now that you know more about the Hygiene Hypothesis, maybe it will help you feel at ease letting your kids experience small and controlled exposures to bacteria/viruses by playing in the dirt, eating sand, putting dirty things in their mouths, etc. since those actions may help prevent them from developing asthma or eczema in the future. At the very least I hope that you have a better understanding about the long-term implications that our changing environments can have on our bodies.