Final answer:
The primary physical barrier the body uses to prevent infection is the skin, which acts as a durable, protective shield against pathogens through a combination of desiccation, acidity, shedding of dead cells, and hosting beneficial microorganisms.
Step-by-step explanation:
The primary physical barrier the body uses to prevent infection is the skin. The skin acts as a continuous, impassable barrier to potentially infectious pathogens. It protects by various means, including desiccation (drying out), acidity, sloughing off of dead skin cells which carry pathogens with them, and hosting beneficial microorganisms that outcompete invading pathogens. Other mechanisms include mucus membranes, producing mucus that traps pathogens, and cilia in the respiratory tract that expels mucus with trapped pathogens. While chemical barriers such as stomach acid, urine, and various secretions play crucial roles in defense, it is the skin that acts as the primary physical barrier.