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There are several epidemiologic patterns that suggest that environmental factors are important in the etiology of T1D.  For example, the diagnosis of T1D is more common during the cold winter months compared to the summer.  This parallels the seasonal patterns observed for infectious diseases, which have been suggested as risk factors.
In addition, when children from countries with a low T1D incidence rate migrate to countries with a high T1D incidence rate, their risk increases and becomes similar to that for the host country.  This difference is much less dramatic for individuals who migrate during their adult years, indicating that the childhood exposures are probably most diabetogenic.
Studies that have compared children with T1D to unrelated non-diabetic children (i.e., case-control studies) have shown that several environmental risk factors are important in the etiology of T1D.  These include infant / childhood diet.  For example, breast feeding appears to be protective, and early exposure to cow’s milk increases T1D risk.  A number of viral infections have also been associated with T1D.  These include those that occur in utero during pregnancy, as well as those that typically occur during childhood (e.g., enteroviruses).  Because the peak onset of T1D is at puberty, it is thought that changing levels of hormones may also precipitate the disease.  Finally, stress has been suggested as a T1D risk factor.  Many parents report that their child experienced a very stressful event (e.g., divorce) just prior to the onset of the disease.
The role of improved hygiene in the etiology of T1D is also currently being explored.  It has been hypothesized that delayed exposure to microorganisms due to improvements in standard of living hinders the development of the immune system, such that it is more likely to respond inappropriately when introduces to such agents at older (compared to younger) ages.  This explanation is consistent with recent reports indicating that factors such as day care attendance, sharing a bedroom with a sibling and contact with pets are protective against T1D. Further studies are needed to determine if improved hygiene can explain the temporal increases in the incidence of T1D worldwide.