Editing Allergy (section)

==Epidemiology==
The allergic diseases—hay fever and asthma—have increased in the Western world over the past 2–3 decades.<ref name="Platts"/> Increases in allergic asthma and other atopic disorders in industrialized nations, it is estimated, began in the 1960s and 1970s, with further increases occurring during the 1980s and 1990s,<ref name="Bloomfield"/> although some suggest that a steady rise in sensitization has been occurring since the 1920s.<ref name="The allergy epidemic extends beyond the past few decades"/> The number of new cases per year of atopy in developing countries has, in general, remained much lower.<ref name="Bloomfield"/>

{| class = "wikitable" style = "width:70%; float:center; font-size:90%; margin-left:15px"
|+ Allergic conditions: Statistics and epidemiology
|-
! Allergy&nbsp;type || United States || United Kingdom<ref name="Chapter 4: The Extent and Burden of Allergy in the United Kingdom"/>
|-
| Allergic&nbsp;rhinitis ||35.9&nbsp;million<ref name="AAAAI - rhinitis, sinusitis, hay fever, stuffy nose, watery eyes, sinus infection"/> (about 11% of the population<ref>Based on an estimated population of 303 million in 2007 {{cite web | url = https://www.census.gov/population/www/popclockus.html | title = U.S. POPClock Projection | archive-url = https://web.archive.org/web/20120516231727/http://www.census.gov/population/www/popclockus.html | archive-date=16 May 2012 | work = U.S. Census Bureau }}</ref>)||3.3 million (about 5.5% of the population<ref>Based on an estimated population of 60.6 million {{cite web | url = http://www.statistics.gov.uk/cci/nugget.asp?id=6 | title = UK population grows to 60.6 million | work = National Statistics | publisher = UK Web Archive | archive-url = http://webarchive.nationalarchives.gov.uk/20021202165044/http://www.statistics.gov.uk/CCI/nugget.asp?ID=6 | archive-date=2 December 2002 }}</ref>)
|-
| Asthma ||10&nbsp;million have allergic asthma (about 3% of the population). The prevalence of asthma increased 75% from 1980 to 1994. Asthma prevalence is 39% higher in African Americans than in [[Ethnic groups in Europe|Europeans]].<ref name="AAAAI - asthma, allergy, allergies, prevention of allergies and asthma, treatment for allergies and asthma"/> || 5.7&nbsp;million (about 9.4%). In six- and seven-year-olds asthma increased from 18.4% to 20.9% over five years, during the same time the rate decreased from 31% to 24.7% in 13- to 14-year-olds.
|-
| Atopic eczema ||About 9% of the population. Between 1960 and 1990, prevalence has increased from 3% to 10% in children.<ref name="AAAAI - skin condition, itchy skin, bumps, red irritated skin, allergic reaction, treating skin condition"/>|| 5.8&nbsp;million (about 1% severe).
|-
| Anaphylaxis || At least 40 deaths per year due to insect venom. About 400 deaths due to penicillin anaphylaxis. About 220 cases of anaphylaxis and 3 deaths per year are due to latex allergy.<ref name="AAAAI - anaphylaxis, cause of anaphylaxis, prevention, allergist, anaphylaxis statistics"/> An estimated 150 people die annually from anaphylaxis due to food allergy.<ref name=Food/>|| Between 1999 and 2006, 48 deaths occurred in people ranging from five months to 85 years old.
|-
| Insect venom ||Around 15% of adults have mild, localized allergic reactions. Systemic reactions occur in 3% of adults and less than 1% of children.<ref name="AAAAI - stinging insect, allergic reaction to bug bite, treatment for insect bite"/>|| Unknown
|-
| Drug allergies || Anaphylactic reactions to penicillin cause 400 deaths per year. || Unknown
|-
| Food allergies ||7.6% of children and 10.8% of adults.<ref>{{Cite web |title=Allergy Facts {{!}} AAFA.org |url=https://www.aafa.org/allergy-facts/ |access-date=24 June 2022 |website=www.aafa.org |language=en}}</ref> Peanut and/or tree nut (e.g. [[walnut]]) allergy affects about three million Americans, or 1.1% of the population.<ref name="Food"/> ||5–7% of infants and 1–2% of adults. A 117.3% increase in peanut allergies was observed from 2001 to 2005, an estimated 25,700 people in England are affected.
|-
| Multiple&nbsp;allergies (Asthma, eczema and allergic rhinitis together) ||Unknown ||2.3&nbsp;million (about 3.7%), prevalence has increased by 48.9% between 2001 and 2005.<ref name="Incidence and prevalence of multiple allergic disorders recorded in a national primary care database"/>
|}

===Changing frequency===
Although genetic factors govern susceptibility to atopic disease, increases in [[atopy]] have occurred within too short a period to be explained by a genetic change in the population, thus pointing to environmental or lifestyle changes.<ref name="Bloomfield"/> Several hypotheses have been identified to explain this increased rate. Increased exposure to perennial allergens may be due to housing changes and increased time spent indoors, and a decreased activation of a common immune control mechanism may be caused by changes in cleanliness<ref>{{Cite web |last=Dablo |first=Rose |date=2022-12-20 |title=Reduce Allergens By Cleanliness |url=https://www.cooperclean.com/cooper-clean-blog/how-to-keep-your-house-clean-when-you-have-pets/ |website=CooperClean}}</ref> or hygiene, and exacerbated by dietary changes, obesity, and decline in physical exercise.<ref name=Platts/> The [[hygiene hypothesis]] maintains<ref name="Hay fever, hygiene, and household size"/> that high living standards and hygienic conditions exposes children to fewer infections. It is thought that reduced bacterial and viral infections early in life direct the maturing immune system away from [[T helper cell|T<sub>H</sub>]]1 type responses, leading to unrestrained T<sub>H</sub>2 responses that allow for an increase in allergy.<ref name=Yazdanbakhsh02/><ref name="Renz"/>

Changes in rates and types of infection alone, however, have been unable to explain the observed increase in allergic disease, and recent evidence has focused attention on the importance of the [[Gut flora|gastrointestinal microbial environment]]. Evidence has shown that exposure to food and [[fecal-oral route|fecal-oral]] pathogens, such as [[hepatitis A]], ''[[Toxoplasma gondii]]'', and ''[[Helicobacter pylori]]'' (which also tend to be more prevalent in developing countries), can reduce the overall risk of atopy by more than 60%,<ref name="Exposure to foodborne and orofecal microbes versus airborne viruses in relation to atopy and allergic asthma: epidemiological study"/> and an increased rate of parasitic infections has been associated with a decreased prevalence of asthma.<ref name="Parasites and asthma--predictive or protective?"/> It is speculated that these infections exert their effect by critically altering T<sub>H</sub>1/T<sub>H</sub>2 regulation.<ref name="Sheikh"/> Important elements of newer hygiene hypotheses also include exposure to [[endotoxin]]s, exposure to pets and growing up on a farm.<ref name="Sheikh"/>