Here's why:

* Physical barrier: The skin's outer layer (epidermis) is made of tightly packed cells, forming a tough, waterproof barrier that prevents most microorganisms from entering the body.

* Chemical barrier: The skin secretes substances like sweat and sebum (oil) that have antimicrobial properties, killing or inhibiting the growth of bacteria and fungi.

* Normal flora: The skin is home to a diverse community of bacteria called normal flora. These bacteria compete with harmful microbes for resources and space, helping to prevent infections.

While the skin is the main barrier, other parts of the body also play important roles in defense:

* Mucous membranes: Line the respiratory, digestive, and urinary tracts, trapping and expelling microorganisms.

* Cilia: Hair-like projections in the respiratory tract that sweep away mucus and trapped microorganisms.

* Stomach acid: Very acidic environment in the stomach that kills most ingested bacteria.

Together, these barriers form a complex defense system that protects us from infection.