What is the Hygiene Hypothesis?

Guest post by Zainab Khan

In most western countries, germs have become synonymous with the idea of something bad that needs to be killed as quickly as possible. However, people have long been questioning the validity of these ideas; a few decades ago it was hypothesized that not enough exposure to germ can and does cause insufficient development of an individuals immune system. New studies have recently shown that this idea of getting rid of all germs, and keeping children exposure to them at an absolute minimum, may possibly cause more harm then good; over cleanliness is suspected to be one of the main reasons that there is such an increased number in asthma and allergy ridden people in western countries. Also, compared to just a generation or two ago, people today have an increased chance of having/developing allergies. Is this all due to society’s craze over germs?

It is important when talking about allergies to have some working knowledge on what happens when an individual has allergies or an allergic attack. Allergies are an extreme and inappropriate reaction by an individual’s immune system to what typically is a common harmless stimuli found in a normal environment; the body takes something such as hay, food, pollen, etc. and has a hypersensitivity reaction to it. The body ends up activating its white blood cells (these are the cells that defend the body against any foreign bad stimuli), which typically are what help humans ward of virus and bacteria, for example the flu or an infection, which results in an inflammatory response. This inflammatory response manifests itself in different ways: asthma, eczema, hives, runny nose or eyes, coughing etc.

The Hygiene Hypothesis

Over two decades ago, the idea that there is such a thing as too much cleanliness was first proposed by David P. Strachan in his Hygiene Hypothesis. The idea behind this theory is that a lack of early exposure to the types of germs and stimuli that people used to have is the cause of allergies. In developing nations and in earlier time periods families tended to be larger then today. It was uncommon to have just one or two children; the idea behind having more children is that the elder child exposes the younger children to more germs and in turn the children end up having to develop a stronger immune system because the immune system has been fully developed by all the early stimuli [1,2]. This idea of exposure to other children has also held true for children who attend daycare at an early age. Daycare children tend to develop fewer allergies then those who are never in such environments. Research has gone even farther to say that children who are exposed to hepatitis A or the measles are less likely to have certain types of allergies [3].

Arguments against the hygiene hypothesis emerged when statistics were followed about inter city African American children in the United States, who have very high numbers of asthma. A study was done that showed that many of these children had been sensitized to the common allergens found around them; however, they still developed asthma at the same rate as those kids who were not sensitized to the same allergens [4]. Also, it is a scientific fact that some allergies have a genetic component. A child who has two parents with allergies has a 75% chance of also developing allergies. There are genetic links that have been found between certain types of allergic responses which complicates the idea of how much immunity is inherited and how much can be developed [5].

Although the idea of germ exposure has been building momentum within the last few years, the debate and research behind it is certainly not complete. If the hygiene hypothesis is true, this opens up another type of debate on how much and what kinds of bacteria, exposure, and caution should be taken around children. What exactly are the “right” germs, and how many are too many? In a society obsessed with antibacterial hand soaps, disinfectants, and bottled water it is going to be quite a challenge trying to convince people that germs are not all that bad.

Works Cited

1. Am. J. Respir. Crit. Care Med., Volume 164, Number 7, October 2001. The Increase in Asthma Ca Be Ascribed to Cleanliness 1106-1107 Link.

2.Strachan David, Thorax. Family Size, Infection and Atopy: The first Decade of the ‘hygiene hypothesis’ Link.

3. Matricardi Paolo, Rosmini Franceso, Riondino Silvia, Fortini Michele, Ferrigno Luigina, Rapicetta Maria, Sergio Bonini, BMJ 2000;320 Exposure to foodborne and orofecal microbes versus airborne viruses in relation to atopy and allergic asthma: epidemiological study 412-417. Link

4. R. Call, T. Smith, E. Morris, M. Chapman, T. Platts-Mills, The Journal of Pediatrics, Volume 121, Issue 6 Risk factors for asthma in inner city children, 862-866. Link

5. Mackay, Rosen, Volume 344, January 2001. Allergy and Allergic Diseases 30-37

Student guest posts: infectious causes of chronic disease

It’s that time again. I teach a class in even years on infectious causes of chronic disease, looking at the role various infections play in cancer, autoimmune disease, mental illness, and other chronic conditions. When I last taught the course in 2008, the students were assigned two writing assignments–to be posted here on the blog. Since this turned out pretty well last time, I decided to repeat the assignment this year; so over the next week or so, I will be putting up guest posts authored by students on various topics under the broad umbrella of infection and chronic disease.

Constructive comments on their posts are appreciated, but keep in mind that they’re students doing this as an assignment and still learning. Finally, these posts are the students’ own; I’m formatting them for publication here, but beyond that their words (and opinions!) are their own.

Post listing:

What is the hygiene hypothesis?

What might have caused my cousin’s nasopharyngeal carcinoma

Post-polio syndrome week: no presidential proclamation required

Cytomegalovirus and heart disease

A look into obesity and gut flora

A rule worth keeping?

Autism and the link to infectious disease

Getting the whole story–attempting to make sense of disease through evolutionary medicine

Parvovirus and Rheumatoid Arthritis–how are they related?

Reviewing the big P…Prions!

Enteroviruses and Type I Diabetes Mellitus

The role of beta-HPVs in skin cancer development

C-sections, allergies, and probiotics

HIV/AIDS prevention; time for change