In the womb, the intestinal tract of the human foetus is microbiologically sterile. However, during birth, the neonate acquires a microbial flora from the vagina, contact with the environment, and through feeding sources. Consequently, a dense, complex bacterial community becomes established in the intestinal tract. The human intestinal flora is a complex ecosystem and may have a number of effects on the health of the host. The flora changes in relationship to diet, age and disease. The maturation of the immune system requires continual stimulation from the developing gut flora. The lack of development has been linked with an increase in prevalence of atopic diseases; the “hygiene hypothesis” (Section 1.6) .