Many microorganisms naturally live on the surface of your skin and hair, and inside your mouth and nostrils including: Staphylococci. Another place where an enormous variety of microorganisms thrive is in your large intestine. These are generically termed Coliforms and include, for example, Escherichia coli. These areas are technically "outside your body" as far as your immune system is concerned - so their presence there does not trigger an immune response.
