Hygiene Hypothesis - Some gardening will help with your gut flora.
Discussion as to any positive relationship between dirt and health has been prompted by a theory known as the "hygiene hypothesis". *The hygiene hypothesis suggests that keeping kids too clean is contributing to the higher rates of autoimmune diseases, especially in terms of atopic disorders (allergies).*
The hygiene hypothesis was first posited by a man named David P. Strachan who attributed the rise of allergies to the fact that family sizes were getting smaller.
In his theory, less children were getting exposed to pathogens through contact from their "dirtier" older brothers and sisters.
Strachan's original hypothesis has been expanded to include the effect of the increased attention to hygiene that is inherent in modern society. Beyond the basics of hot water and food safety practices, the list would the use of antibacterial cleaning products, the increased frequency of antibiotic prescriptions, and the use of vaccines. Although not specifically studied (to my knowledge), along the lines of the "fisherman's theory", kids no longer spend much of their time playing outside and getting dirty.
The hygiene hypothesis holds that these societal changes have reduced the likelihood that children are being exposed to the types of bacteria that cause infections at an age where their bodies would be programmed to develop immunity. When exposed to such pathogens at later ages, the body is less equipped to deal with them, and ongoing problems ensue.
It is thought that the protective element of early childhood exposure to pathogens affects the development of cells within the gut associated with the response of our immune systems. There is some limited evidence that the makeup of the gut flora of children who suffer from allergies is different from those of allergy-free children.