Chicken viruses and leukemia

A lot of us suspect that there is a link between immune system problems and viruses or chemicals.

But this study of an increase in blood cancers and chicken farms brings up a question of viruses causing the problem, and does note another link:

Growing evidence suggests that kids raised on farms are less prone than non-farm kids to asthma, allergies, and other conditions involving the immune system. Some studies have found that people who suffer from these conditions are actually less likely to develop cancer than nonallergic people. One explanation for this, as the paper notes, is that "allergic conditions may enhance the ability of the immune system to detect and eliminate malignant cells." It'd be interesting to look at childhood farm exposure and cancer rates in the developing world, where asthma and allergies are much rarer than in the US.